Yeah, artists usually make a call/plead for help for guest comics during chapter breaks, and it's best to follow and love the artist you're wanting to make a guest comic for. I usually make my call for guest comics over twitter.
However, fan art is awesome, and all creators love it! You could probably approach the artist you want to draw a guest comic for, at any time, and say something like, "Hey, I really love your series. Like a LOT! To the point where I want to take on extra work and make a guest comic for it, cause I that in to it. Would it be cool if we could set this up? Maybe you could use it as filler inbetween posts to take a well deserved break or something?"
I have a feeling most artists, getting a message like that, would respond with, "OMG :tears of joy for days: That'd be super awesome and I'd love that immensely! Let's totally work this out and plan a schedule where I can post this and take a break to refill my buffer, you beautiful creature you! :hearts hearts smiley face hearts:"
:everyone turns into a valley girl in my reenactment:
So yeah, after contacting them and getting the ok, present your strip idea to them (if you already have one in mind). For the most part, if you're approaching the artist first anyways, I'd assumed you'd already have an idea of something you'd like to do for a gs. I generally don't like asking the artist for an idea for the gs.
A guest comic is my way to show my love and appreciation for their work and asking them to provide an idea feels like it might diminishing that specialness or taking a potentially idea from the artist that they would have used in their own series. idk, maybe that's just me though, if I don't have an idea before hand I don't volunteer.
So yes, in conclusion Ask, Get Permission, Send Idea, Do the GS with lots of Love, and Send it Back, Done. Don't do it for the exposure, do it because you love the comic!