To sponsor a memorial costs $5. I usually use my Visa card, there are other options. You can pay online or you can send payment through the mail. Once a memorial is sponsored, there will not be any more ads on that page. (Any not famous memorial can be sponsored, not just fallen gravers.)
On the memorial page along the left side of page is a blue section. The Find a Grave logo is at the top of column, then a box that has a list of "actions". Below the box of "actions" there should be a pink heart that says "Sponsor this memorial". Click on that heart and follow the instructions.
If you "Refresh", or use the "Back button" after clicking the submit button while adding a memorial, please check to see if a dupe was created. (Dupes don't always show up immediately.)