This is how I made a Jurassic park inspired T-Rex head.
Step 1: Rough Shape
Start by making the rough skeleton shape of the T-Rex head out of cardboard, using a hot glue gun to stick the pieces together. Make sure to use plenty of reference pictures from a variety of different angles to make sure you get the correct shape. Using pvc pipe or pieces of wood to give the head a stronger structure and hold the heads shape.
Step 2: Strengthening the Structure
To strengthen the base skeleton shape of the T-Rex head, apply multiple coats of resin over the entire structure. Though when applying remember, the more layers of resin you apply, the heavier your finished product will be. Fibre glass mat or cloth can be added onto the cardboard between the layers of resin to give it extreme strength, though this will also add a lot more weight to your T-Rex head. The lighter the finished piece is, the easier it will be to mount it onto your wall. A good cheaper alternative for using fibreglass resin would be simple paper mache. This method of strengthening would not be as hard and strong as the resin though would be lighter.
Step 3: Adding Flesh
Upholstery foam of different widths will be used to create the flesh and skin of the dinosaur head. The foam will be added to the skull structure using spray adhesive. Use plenty of reference pictures from different angles to see the different shapes you need to make with the foam.