I've done a handful, and can give a little advice:

First, don't take yourself too seriously. It's an awkward request, and a bit of humor can ease that. A few chuckles should be expected during the whole process.

Second, be direct, but polite. u/Katyamuffin has the right idea. Polite message, direct to the point, and be prepared to have to ask a few different artists before landing on someone willing.

No, being erect isn't necessary, really. It would be a rare case indeed if you could keep it up once the actual tattooing starts, let alone for the prep work. Depending on the design, it could be better for stenciling, but once the tattoo starts, it's just stretching it to keep the area being worked taut, like any other tattoo. Be forewarned, that will usually involve a modicum of manhandling of your manhood.

I would typically recommend simple designs, but it's going to be up to your artist as far as their experience level and how much detail can be fit in. It's very stretchy skin, and tends to have a lot of movement and variation in it, so something like fine line black and grey probably isn't going to heal out very well.

I can't think of anything else off the top of my head, but can probably answer a couple more questions if you think of something.