This is (or was) one of the best tutorials on this that I've seen -- sadly the images have all evaporated, but the info and processes are still very good:
http://www.britishblades.com/home/articles.php?action=show&showarticle=98
One of the most important points is that you should use a clamp/vise to squeeze and compress the stack down when you're doing the glue-up and assembly - treat it as a separate part of the assembly -- don't try to just use the end pieces of the handle to hold the stack as it's being glued - the resulting leather handle won't be nearly dense enough and it won't last long.
Edit to add: just did a Save Page As... (Webpage complete) from Firefox to my local drive and got the whole tutorial - pictures and all. Weird, but very handy.
-- Dwight
Yep on the compression and square leather pieces instead of rounds. Compressed wet formed washers will not loosen or move during weather changes, as well as loosening with use.
1. Use vegetable tanned tooling leather and water soak the pieces for a couple of minutes.
2. Having the leather washers with their tang slot holes cut before soaking and fitting on a long bolt is a must.
3. Using a long 1/4" or 3/8" or so diameter threaded bolt is going to be your compression vice. Along with the bolt, you need two large washers at least the size of larger than the leather washers.
4. You are going to need quite a few more leather washers than you probably first realize. The compressed handle will need about a 1/4 to a 3rd more washers than a loose stack of dry uncompressed washers.
5. After the square wet washers are strung onto the bolt (one steel washer by the head of the bolt to act as a stop and the other steel washer at the other end of the wet stack by the bolt nut) tighten the nut down to where you want the finished thickness of the leather washers. Compress the leather washers down to half or 3/4 of their original thickness. This is very important to compress as mentioned.
6. After the bolted together leather washers are dry, remove the end bolt nut and slide the washers off the bolt. Be careful with the dried stack, keep them stacked EXACTLY the same as the dried stack comes off the bolt. This shouldn't be a problem because the leather washers are wet formed to each other and tend to stick together.
7. Contact surfaces of the wet formed dry washers then need to have contact cement applied to contact surfaces. The contact cement takes just a few minutes to dry.
8. Knife tang needs plenty of epoxy applied to the tang before the washers are slid on. Remember, the washers have to be stacked exactly the same as they were on the bolt when wet formed. I use Brownells AcraGlas gel on the tang and use enough to cover the tang and also fill in the gaps of the leather washer tang hole.
You can do one washer at a time on the tang and fill in the tang hole voids, then add the next washer, etc. Don't worry about excess epoxy, it will squeeze out when the handle gets its final clamping. Any excess that squeezes out of the stack acts as another washer lock. The 'Glas has an hour working time and you have plenty of time before it begins to set up.
I use a simple homemade clamp to do my final clamp on the knife handle. The final clamping doesn't need a whole lot of pressure, just regular clamping pressure because the leather handle stack has already received clamping advantage when wet formed. The wet formed leather washers lock together really good by themselves when stacked on the handle in the original order because of small natural pits and wrinkles in the leather that will match up perfectly together. You will see this when you assemble the washers onto the tang.
When the freshly glued and epoxied stack is clamped, you will notice the excess epoxy squeezing out several exits. This is natural and just fine. Just wipe off the excess. I use a piece of old plastic shopping bag or food plastic wrap between my clamp jig so the works won't be glued to my jig.
9. After epoxy is dry, I remove the knife and grind the handle on my belt grinder. Great care is advised here. The grinder belt will eat leather like nothing else you have ever ground. A light touch is advised!
The reason I use square leather washers is for a grinding index. It gives me an index to keep the roundness of the handle in proper prospect.
10. Finish the leather as desired, and apply a good sealer.
The whole idea of the wet formed leather handle prevents the washers from loosening in the future, and the epoxy will lock it onto the tang, forever solid.
I apologize for no pictures, but this is the best I can describe the process. It sounds like a long process, but it's quicker than it seems.
One other suggestion, you could do that I haven't tried yet, skip the contact cement and use the 'Glas gel between the leather washers when assembling the washers on the handle.