Always use safety glasses when using stamps. Stamps are hardened steel and may shatter, even when used by someone with years of experience.
Begin with a light hammer. You can always go deeper by lining up the stamp with the first impression and re-striking.
Practice first. Use your new stamp a few times on a practice piece first before stamping your finished products.
Never strike the face of the stamp with a hammer.
A controlled, even swing will give the best results; do not try to overpower the stamp.
If a stamp chips or cracks, do not continue using it. A cracked stamp may break from the stress of being struck, even when stamping soft material.