I have done a few... Formulin sourced from a pharmacist, which is basically Formaldehyde and water. Inject it into every crack in the skeleton ensureing that the cray is pumped fuller than full with it.
If you look at the knuckles on your fingers imagine they are the same as they joins in the exoskeleton on a crayfish, the formulin must be injected in both directions into every joint, I found satrting at one end and working towards the other, then turn the crayfish over and repeat, at the tail it pays to inject in both directions on either side of the tail to ensure its full!! If you dont get it into every part of the crayfish it will start to rot and smell as the formulin breaks down the protein in the crayfish, and it can take years for the smell to reduce.......
After it has been completely filled up set the crayfish up on a large flat piece of wood in what ever pose you choose. I tended to have the feelers rolled backwards as they tend to get broken if left extended outwards, i tied these down with some mono when i was happy with where they sat. I put a small block under the chest so the cray looked like it was up on its haunches, with the legs spread evenly outwards in a fan fashion. I drilled small holes in the piece of wood I set it to dry on, and placed the tip of each leg into the holes so they dried where I wanted them to. I pulled the tail out straight and fanned the end of the tail outwards and used dress pins to tack them into place, again so they dont move as the flesh dries and constricts.
Place the drying crayfish in a warm dry spot out of the sun away from prying eyes and hands and wait 8 weeks...... I made a shelf in the garage above the garage door.
After the cray has set and dried, you will know if you didnt get the formulin into every crevice ... If you get it right the cray wont smell.
I coat em with a uv safe polyerethene as sun light is the biggest enemy, causing the cray to fade and become even more brittle.
As many coats as you like, and the cary will look like it just came out of the water.
Make sure you wear a long sleeve top, rubber gloves and glasses as the formulin should shoot out of the holes you have created in the joints, getting it in your eyes will not preserve them, the stuff will dry out any skin it comes into contact with. Getting it from a Pharmacist can be problematic.
Good luck