|I save those slivers of soap that usually go mushy in the shower or handbasin, as it seems a waste and bad for the environment to throw them away.
I just dry them on a flat surface until they are hard again,
use them in place of chalk to trace around patterns using the common method of placing the pattern on the material, using cans or weights to hold in place, then trace with the soap and cut out the material.
I have used this method on micro fleece, t/shirting and w/cheater material with great success . Moisturising soap might be risky though. Also I have only tried white soap.