I have done this with plates before, I simply list the number of plates she was ready to pay for and I noted in each listing which payment it was, etc. so she could easily keep track as well. You just need to agree on a payment plan with your buyer, and then list each payment with all of the info for you both so you don't lose track of where they are at, depending on how many payments they are doing. You can call it Layaway, or whatever you want to...
Don't forget about shipping though, as you don't them overpaying on that or underpaying either...
Hope this helps.