First, you'll want to calculate the volume of the mold to determine just how big of a soap recipe you'll put into it. You'll need to line it before you pour soap into it. Use freezer paper or recycle the sturdy wax paper from your cereal box. Put the mold together, but remove the end pieces. Place the liner in the mold so that it conforms to the sides and bottom. Wrap the end pieces before placing them in the lined mold. The end pieces can either be wrapped tightly with saran wrap, or shrink wrapped. If you decide to shrink wrap them, the wrap will last through many batches of soap.