... run your index finger over the stamp ...
But make sure that you:
1) wash both hands with soap & warm water, so you do not deposit dirt on the stamp, and
2) place the stamp on a perfectly flat surface, so you do not curl it, and
3) dip the intended index finger in alcohol, to remove/destroy any oil on your finger tip's skin, and
4) use a clean, dust-free hair dryer to evaporate any residual alcohol, and ...
... and I think the archival threads are getting to me.
Meanwhile, thank you, disi123, for the protocol.