1. Did you find a place/person/organization of interest?
How did you find this out? What clues do you already have? Are they from credible sources? Do you have an address? A name? Having both is always better.
2. Were there really here?
3. Do you have the permission?
contact known family members, estate managers, etc. - ask for their blessing
contact your local arts council, city council, mayor, etc. - let them know of who you are and your intentions
contact current residents and/or owners of the property - snail mail and/or ring their bell
4. If people are not responsive:
call, email, snail mail, ring their bell again
last resort: put the plaque up with all your contact information sent to all people involved and wait for their response (Be nice. Make it easy for them to remove if they do not like it. Never damage their property.)