If I felt things were going well between me and and attending I just sent an email near the end of my time with them asking if they would feel comfortable writing me a strong letter. Attendings are busy, no need to set up a meeting just to ask for a letter. Email is just fine. When they responded I asked them if they wanted anything to help them write a letter ie. meeting, CV, personal statement, etc.
My advice: Ask as soon as you finish your rotation, even if it's early in the year and you won't need letters for months. Once you get the initial committment, then it's easy once application time comes around to drop off a package with your info gently reminding them to get the letter out.