1. You contact who ever you want to MM for you (Philly, Quad, Christy, etc.)
2. You get all parties involved into a group chat
3. Inform the MM what the deal is and who is giving what to whom
4. You send necessary info to the MM (PRIVATELY NOT THROUGH GROUP CHAT)
5. MM secures account or whatever your are trading/selling
6. MM informs the other person he has the account secured so he can pay you (Or you can have the MM get the money and send it to you) or send the MM the info the account traded
7. After securing all accounts and whoever gets is getting paid gets their money the MM will send the account to the new owner(s)
Best/most detailed description I could give hope it helps