Washer stamping

I know a lot of people here had stamped washers bought in a hardware store, so I'll ask here:

How do you get a clear letter stamp? I hammered those things really hard and barely made a dent where you cannot really distinguish which letter it is. I tried both sides and nothing. What am I doing wrong because I've seen tutorials and people barely apply force, they just tap them twice and voilà!

Ps: i bought stainless steel flat washers and I'm using a professional Steel Metal Stamp Set.

EDIT: Thank you everyone for all the great advice. It worked!!! I ended up getting a 3 pound hammer and a small anvil.