Simply comment anonymously with a fic scenario you'd like to see written (your request does have to relate to the Doctor and the Master in some at least tangential way), and respond anonymously to other's requests with anything from a drabbles to full length stories. If you make a ton of requests? You should definitely try and fill a couple. As Mama in Chicago would explain, it's about reciprocity.
Give it a try! Play hard, have fun!
Edit: *If you really like a prompt and it's been filled, feel free to respond again! One prompt can spawn multiple ficclets!*
Return of Edit: If you don't necessarily need them to sex right nao, but just want to see them do some specific thing, like talk about their granddaughter Susan just as an example (pulled from an idea below), that's awesome as well! Non-porn on the Anon Porn Meme! So Transgressive!
EDIT OF RASSILON: Write RESPONSE in the subject line of a request-fill post to make navigating to responses/telling what's been answered vs. just ditto'd easier for everyone. I don't want to hate on ditos/expansions/qualifications, they're useful! But threads collapse all annoying-like with this layout. x_x
EDIT OF OMEGA: Recently we've had some kinda dubious anon meme posts. The prompts in question are fine; some prompts are responses to others, and you're allowed to have opinions. But exercise best judgement/play well with others. Don't diss other people's prompts when you make yours. You may be anon, but this ain't who_anon. Not everything's gonna be your cuppa. It would be nice if we could avoid 'your choices are invalid, this character is this way' in the future.