Two nine-inch round cakes, frosting, decorations, and an unquenchable thirst for adventure are all you need.

Step 1: The Raw Materials

Two nine-inch round cakes. Cakes made from scratch will be denser and stronger than a cake mix, but you love the Duncan Hines don't you -- just be forewarned that the time you save using a mix will be spent cursing during the frosting crumb coat.
Two batches of frosting. We did a butter cream with 50/50 butter/vegetable shortening. Die one batch green. Keep 1/2 of the second batch white and die 1/2 blue.
Rolled fondant. We don't know what this stuff is, but we were darn sure we weren't going to make it. Found some multi-colored fondant at Michael's, a big box craft store. Buy the non-stick mini rolling pin while you are at it.
Decorations: cinnamon red hot candies for the eyes, candy corn for the tail spikes, chocolate chips for toenails, and toasted coconut for the prehistoric grass.
One cardboard cake board, half-sheet size.
Frosting pipe tips and bags. Use a star tip. Yes, this makes a difference, so don't skimp here. Really.
A partner. Not required, but could speed things up during frosting or at least help pass the time.

Step 2: The Body

Bake the cakes and cool completely.
Take out of pan, and find the center of one cake.
Cut the cake in half with a bread knife.
Put the two halves together with cuts edges aligned, and place on a work surface as pictured.

Step 3: The Appendages

Ever heard the advice "measure twice, cut once?" Unless you are a blackbelt paleolithic pastry chef who can think in three dimensions of cake, you will probably want to make a paper template to design the head, legs, and tail. I drew the template below, and you generally want to cut the paper in the same shapes I used. Mouse over the shapes for a description.

Cut a 9-inch circle of paper and lay it on top of the second cake. Draw the tail and head in one half, the legs in the other.
Cut out the paper pieces and arrange them on your body segment to see how they look. Tweak and repeat as necessary.
When you are feeling lucky, carve 'em and stack 'em. Oh, and you may want to fix the head to the body with some toothpicks. Just in case the birthday party gets a little rowdy.
Move all pieces to the cake board.
Use white frosting to join the body halves together, then join the appendages to the body.
Trim the corners and square edges off the feet and shoulders if you like.

Step 4: The Skin

Roll the fondant out 1/8 inch thick. Hand cut the fondant with a small knife some diamond shapes for the plates on the dinosaur's back. Detail the plates with a toothpick to give them a ribbed texture.
Spread a thin layer of green frosting over the cake. This is called the crumb coat -- and for good reason, as much of the crumbs pull away and get mixed up in the frosting. Use a very light hand on the cut surfaces of cake. Did you use a cake mix instead of making one from scratch? Are you cursing now?
Tip: crumb coat one half. Have a partner start piping with the star tip while you finish crumb coating the other half.
Use a star tip to pipe on green frosting, then blend in blue for an accent.
Stick fondant diamond plates into back. If the frosting does not hold the plates well, stick a toothpick into the plate then stick it into the cake.

Step 5: The Final Touches

Spread a layer of white frosting on your cake board and toss some toasted coconut around for grass.
Add any other finishing touches you like. Candy corn for spikes on the tail, cinnamon candies for the eyes, and chocolate chips for the toes. We made palm trees using tube cookies with fronds of parsley -- this was the first thing every kid wanted so make a small forest if you do it.

Grab the camera and enjoy!