Thumbprints are a state rule, not a federal one
Thumbprint rules are set by each state. Many states do not require one, so check your handbook first. See what goes in a journal.
Check your state's rules
Some states require a thumbprint for certain documents, while others do not require one at all. Ask your state notary commissioning office or check your handbook for the rules that apply to you.
Inkless pads and phone capture
Whether an inkless or phone-captured print is accepted depends on your state. Actbook has a thumbprint entry feature, so ask your commissioning office first.
If the signer cannot give a thumbprint
Your state may give a procedure for this. Check your handbook.
Ready to keep your entries in one place? Get the iOS app for Actbook, a notary journal and fee and mileage log for iPhone.
Not legal advice. Check your state's notary handbook or commissioning office before you rely on any summary here.