‘Put’ is better than ‘attach’ or ‘affix’ but it’s still not enough to be clear to me, because we have no experience of doing anything like that. You would normally include more instructions, for example Place an imprint of your right thumb in the box below. Use black ink ….

Celia, 'fingerprint' could refer to the layer of skin, or to the mark on the paper by the ink and the impression. Perhaps I'm wrong.

How would we 'attach' our fingerprint? It is a layer of skin. Shall we cut our skin and give it to you? We could 'make' a fingerprint on the paper, we could 'press' our finger on the ink pad and then on the paper.