The first thing that you need to do is find a template that you like. You can find quite a few with a simple Google search. My favorites are these two by instuctacles users kaptaink_cg and stevemoseley:
http://www.instructables.com/id/Build-a-6-0-tall-Wooden-T-Rex-Model/
http://www.instructables.com/id/T-Rex-Dinosaur-Puzzle-with-different-sizes-and-pos/
Stevemosley actually made his from scratch by scanning a picture of a T-Rex skeleton and modeling it with CAD software.
Another way to get a template is to purchase a small 3D wood puzzle from your local craft store, then scan it and enlarge it to the size that you want. I have included several patterns in Step 8 that I scanned from puzzles that I recently purchased.
In the end, I decided to use a pteranodon template that I got from Kaptaink_cg’s instructable.