This instructable details the making of a soft quilted tetrahedron costume for the more patient of my two cats.
Step 1: Choose fabrics and preprocess them
You'll also need an attractive color of quilt binding. Binding can be purchased at most fabric stores (I use Joanne's). The binding is a cloth strip that will sandwich the outer hem to create a finished edge. Also make sure you have a color of thread that matches your fabrics.
Step 2: Cut triangles and piece them together
Measure a strip of your 3rd contrasting fabric for the outside edge; the border shown in the pictures is a 2" strip. When the 4 triangles are sewn together and ironed into a bigger triangle, pin the border on the edge and sew in place. Once one edge is attached, cut the edges flush, iron flat, then repeat for the other edges.