OK, so  you've got the knife taped properly, you have a design in mind, and you have the handle materials purchased. Now the fun begins.
WORK AREA PREPARATION
As a prelude I recommend you clear your workbench of all extraneous tools, etc. so you can do an efficient and proper job.. The goal here is to get everything as ready as humanly possible so you can reach everything you will need quickly. You should have some paper towels neatly stacked in a pile where you carefully tore them from the main roll. You should have a metal can and lid with some acetone in it for cleanup and dipping of the paper towels. A pair of nitrile gloves( buy them in a 100 count package from a tool supply place) for you hands is also nice if you have problems with acetone or the epoxy resins. Small pieces of wood or old screwdrivers or probes are also handy for pushing and pressing epoxy laden pieces around. A sheet of wax paper taped in one corner of your granite block is a great work area for the knife.. Epoxy is sure to drip off the knife, but it will not adhere to the wax paper.
PIECE PREPARATION
Before I ever squeeze out the epoxy onto the granite block for mixing, I have all my pieces laid out at the top end of the block in the order and orientation,they will be installed. I have learned this from much practice and many mistakes. When the epoxy is mixed, the clock is running, and that is no time to try to figure out which way you meant for the pieces to be installed.
The width of the pieces and spacers must all be correct and ready. This means you have to do some cutting and sanding by whatever means to get the correct width for each and every piece. In addition, the pieces MUST have parallel faces or properly done angles if you are using triangular or other oddly shaped pieces. The length of the piece should be such that there is a definite overhang at each end of each piece when installed.  This is a must, because you have to be able to see that each long end has been placed so that it can be cut and sanded back later  There is no way to add material after the fact for a piece that wasn't long enough to begin with, or slid slightly on the wet epoxy after installation.
One techique I use for a knife that has a front bolster is to install all the pieces at one time, except the last one in the middle. Whether the knife has an actual back piece or is just open, you can still use this technique for centering. Of course you can also install the pieces one at a time from the bolster back, if symmetry is not an issue in your design. If you have a symmetrical design with multiple pieces of different widths it is best to do both ends towards the middle because the last piece can then be sanded exactly to fit snugly later. This will require two epoxy mixings however, as I will explain later, but will result in a finished piece that IS symmetrical. On a custom knife handle, not doing this will result in a handle that will look unsymmetrical and out of synch.
If you intend having both sides of the knife identical (even with different materials like I do) then it is best to do the cutting and sanding for both sides before every gluing the first side. One of the things I look for when I inspect someone else's custom work is alignment. By that I mean how well the spacers and pieces match between sides, as viewed from the top of the knife looking down on the spine. Matching intersections are, to me, the sign of a craftsman and quality work.
To accomplish all this precision you should have, as I believe I mentioned before, a reasonable set of calipers. The new digital metal calipers are not all that expensive and work quite well, and besides they have many other uses besides the knife work. Plastic ones are inexpensive and work quite well also. A nice tool to have around. This tool will quickly check how well you have sanded the individual pieces and kept the edges parallel and just as importantly will make absolutely sure you have the same width where you want the widths to be the same.
One last sanding item, and this one gives me fits sometimes.  It is obvious that all pieces must have a 90 degree angle between the bottom face that attaches to the handle and the two sides (unless some special design is being done). For me this is one of my most difficult tasks! I invariably end up hand  sanding the sides and must be constantly watching  that I  hold the piece vertical while sanding. You don't need any aids to see when you haven't done the job correctly, and yet it is essential to have these edges parallel and 90 degrees so you get that perfect fit.  Take your time with this step and make sure all the pieces are correct. The epoxy flow will cover some very minor errors, but it's always best to do it right. Go slow and view often!!
Two other minor items need attention also.  The bottoms of all pieces must be flat, but roughened enough for adhesion with the epoxy, and all pieces should be carefully given one quick wipe with  acetone or alcohol to remove any sanding dust, then carefully placed in their assembly order as mentioned before. All I have said for pieces includes any thin metal spacers. Make sure the metal spacers are pounded flat by doing both sides with a ball hammer and pushing outwards.with the hammer in a smoothing fashion. The granite block will let you knwo when you have  them flat.
The height of the pieces above the final finished level doen't really matter other than the amount of sanding you will have to do later. You should error on the plus side for height. Again, you can't go back and fill if the piece wasn't tall or thick enough. Finally, make sure bolster edges and spacers are sanded and roughed up slightly.
MIXING & ASSEMBLY
It is almost ridiculous how fast the knife finally comes together when the preparation has been done properly. I love this part even though it is somewhat messy.
With all tools and pieces in place, it is time to mix the epoxy. Squeeze or measure out equal parts
of resin and hardener onto a corner of the granite block, and start mixing with a small stick that you keep for just this purpose. Error on using too little epoxy, since you can mix more if you need it. Make sure you spend a FULL 60 seconds folding and swirling all parts of the mix or you will end up with portions of the handle having epoxy that just won’t set up.
With the mix in a liquid heap, take the knife and swab epoxy on the portions where the pieces will be installed. Make sure to swab a little on the bolster face(s) that will touch a handle piece. Now put the knife down and make sure it is steady and level on the wax paper. Now carefully take one piece at a time with some forceps or your fingers and swab the bottom and both sides lightly with epoxy, then lightly press into place. The epoxy is sticky enough to roughly hold the piece for now.
Continue swabbing and stacking pieces until you meet in the center with the one final center piece to go, or you have finished back to the end of the handle. As you install each piece, lightly press it down and push it against the previous piece. In some cases, if too much force is used, the previous piece (especially spacers) will pop out slightly due to hydraulic pressure. If this happens, use one of your very thin needle tools to gently push the piece back down.
Now is the time to quickly wipe your mixing and swabbing stick clean, with first a piece of dry towel, and then with an acetone soaked piece of towel. You can use both these towel pieces to also clean the granite surface.
If you have installed all the pieces on one side with out without the center piece in a reasonable time of say 5 minutes, you still have plenty of time on the 30 minute epoxy to adjust the pieces.
I carefully lift the knife off the wax paper, holding it absolutely level so nothing slides around and then I start checking for fit. This is easier written than done, so be prepared for problems.
First I look to see if the pieces are extended far enough over the liner they rest on, for both ends of the piece. This is especially true for spacers. You will need the Optivisor for this because epoxy will be everywhere and you need the magnification to see if thin pieces are properly positioned. Next check that the pieces are all butted snugly together. If there is a small space anywhere, try to push pieces against it to close it. Small errors will disappear because epoxy will fill them and sanding will smear material around, especially metal spacers. Remember to keep the knife level or you will be sorry. As the epoxy begins to set you will still be able to move the pieces, but slippage won’t be such a concern. Keep checking for proper alignment by going around again to see if all pieces have their bottoms close to the liner material and haven’t moved. You can’t check enough. Once you are satisfied, put the piece carefully back down on the wax paper and make sure it stays level.  Clean up your hands and any of the tools that have epoxy on them.
Come back in ½ hour or so to check the progress. Pieces should now be stuck in place and touching any epoxy is a very sticky operation. If you are really good you can take a lightly moistened swab and clean out the epoxy from the missing center piece. If You don’t, you get to scrape it off the liner before fitting the final piece. And, don’t forget to NOT swab the final pieces on each side of the missing center piece or you get to scrape those surfaces also.

That’s enough for one day.

Dr.  K