While searching for "how to draw a D20", I found a tutorial on PatchworkPaladin.com. It demonstrated how to get the basic shape of a D20 in 5 steps. I have made some minor tweaks to the original steps. Here's how I do it now.

Step 1

Draw an M with the bottom point a little longer than the legs.

Step 2

Add two lines going up from the sides of the M's legs a little longer than the two top points.

Step 3

Create triangles by drawing lines to connect the points at the top and bottom.

Step 4

Draw a triangle up from the center triangle and connect the point down with a line on both sides to make two more triangles.

Step 5

Add a line down from the bottom of the tip of the center triangle and connect the points to form two triangles.

Ten triangles later and you are done. Only thing left is the numbers and/or decorations.