For finer details of recessed designs, try putting a piece of paper between the metal and your block. Not enough? Fold the paper. For some, thicker backings can help, like cardstock or leather.

For raised designs, be careful to not use too thick a backing, so the edges don't impress. Raised designs are generally easier to stamp.

The surface you strike upon MUST not bounce. Consider finding a stump and investing in a large block or anvil.

If you're used to using a 1# hammer, this will only work with the smaller stamps. For larger stamp surface, you'll need more force to drive the stamp into the metal. It's not uncommon to use a 2#-4# sledgehammer. Consider buying Brass if you don't already own one. They're more gentle on the stamps.