I'm sure there is some mathematic equation that will tell you how to get a 3d cone form from a 2d nipple diameter. Barring discovery of this formula, guesstimate.

Take a circular template slightly larger than your nipple diameter and cut it out of the craft foam.

Cut a "Pac Man" shape out of the form. Be sure your first cut goes to the middle of your circle. If you want tassels cut a small portion out of the center for the grommets.

Hold the two sizes together to make a little hat shape. Check to see if it fits you. Trim as needed.

Sew the two sides together.

Make four of these. Two of them will be against your skin so take care that your seams are as smooth as possible (we'll refer to these pieces as the bottoms). The other two we're going to sew your fabric on (we'll call these the tops). The bottoms should be slightly smaller than the tops and don't need grommet holes.