There's no official way to ask, just ask. They've been through it themselves, both as students and as preceptors.
As mentioned, give them a CV and if you'd like, a personal statement too. That way they can write something personal about you even if you only worked with them for 1-2 hours/day for a month.
The only official thing you need to give them is the ERAS cover sheet with the instructions (letterhead, where to send the letter, the waiver of your right to read it, etc...) to the preceptor. The cover sheets are available after you register for ERAS (note: register for ERAS, not submitting the ERAS application.) Ideally you would have already asked your preceptors from whom you're soliciting a letter by the time you register, or at the very least know who you'd like to ask.