There are multiple templates available online. However, for a custom size, it is easy to create a template. Draw a pentagram the size of one side of the finished solid. Trace the pentagram in the middle of a piece of cardstock. Draw another pentagram on each of the five sides of the first, creating a group of six. Repeat these steps again so there is a second group of six pentagons. When assembled, there are 12 pentagons in the shape, one on top and one on bottom with 10 others around the middle.
Cut the groups of six or the template shape from the cardstock. Cut carefully so the edges of the finished solid are sharp and straight.
Use a ruler to make the folds sharp and crisp. Lay the ruler on the line to fold, then bend the cardstock up. Rub your fingernails along the crease. Continue to do this for all the edges. If the pattern used has tabs to make gluing easier, fold them as well.
Tape or glue each of the five pentagons surrounding the original one to its two neighbors, then to its corresponding piece on the other pentagon group. If there are tabs, fold them inside the shape and glue or tape. If there are no tabs, tape the pentagon sides together on the inside or outside. Continue assembling until all 12 sides are attached. The last tab might be slightly difficult, depending on your method of assembly.