In this pic, you'll see where the red arrow points there is a "V" or "W". Use a knife or chisel and cut it.

Then you need to make a cut all the way around the bottom piece of the shell leaving just enough skin to stitch back together when you mount it as shown with the blue lines.

Then you can take the bottom off and dig all the goodies out of the shell. I like to cut the bones at the joints where the legs, neck and tail come off so you can work on them.

The tail can either be tubed out or split to the tip. It's up to you.Then tube out the legs and neck. Be careful skinning the feet. Try to use your fingernail and a blade to turn down to the toes without actually pulling the skin because it can rip all the way to the toes and then your in trouble.

When you tube the neck out stop when you get to the skull (it stays in) and cut it off. Then get as much meat out of the head as you can from the back (I use a cheeker tool to do most of it). Turn the neck back right side out an use needle nosed pliers and brake up the top pallet and get it out. Then dig out the meat and eye's. Pull the tongue out and flesh it good. There is some meat in the cheeks that needs to come out too.
Flesh everything out really good and break up the spine which will be attached to the shell and dig out the meat and spinal tissue. Wash with Dawn and water and toss it right in a pickle.
This is NOT an easy job to do and I would STRONGLY suggest getting Tom Voyer's DVD on mounting a snapping turtle that can be purchased from either Joe Kulis or WASCO and it's worth every penny. Everything I've learned about turtle mounting was from this video.
http://www.kastaway.com/store/store_product_detail.cfm?Product_ID=173&Category_ID=1&Sub_Category_ID=12Hope this helps.