Follow these suggestions for successful stamping:
To create an impression in soap, center the image end of the stamp on the
soap then lightly tap the wooden end of the stamp with a hammer 2 to 3 times. A
regular hammer works well, but wooden or rubber mallets are gentler on the wood.
Practice on scrap soap to get comfortable with the process.
A flat soap surface allows for even, consistent images. You don't need to
stamp too deep- about 1/8 inch is plenty. The deeper the impression, the more
soap is displaced, which can warp the outer edges of the bar. Smaller bars can
be stamped to a lesser depth to avoid warping or breakage.
Depending upon the formulation of your soap, the soap may be ready for
stamping as soon as it's unmolded. Softer and/or superfatted soaps may require a
few days of aging. The soap should not stick to the stamp; if it does, the soap
needs to harden a bit.
Stamped designs show up best on colored bars.