Speaking from experience, and I've been doing this for years, you simply need to desensitize yourself to it. And I still get excited when when I go to change my diaper. It's just there and you have to deal with it.
The simplest solution is to point your penis toward your belly-button and simply diaper over it, snugging it up nice and tight. You can't void your bladder with an erection anyway, it's physically impossible. So at this point you have one choice, get distracted by something and it will go away. At which point you excuse yourself to the restroom and tuck it down properly. At this point you should re-adjust your tapes around your waist to give a good fit.
Generally when you diaper you want to make sure it's good and snug around your legs. Even if it's slightly uncomfortable at first, but keeping your leg cuffs nice and snug prevent leaks in a big way.
Initially I reccomend learning to diaper while laying down, it's the easiest position to work from. Open the diaper up all the way. Stretch the elastics out and fold the wings out. Either sit down on it or pull it under your bottom. Pull the front up between your legs, and check the fit. The top edge of the front of the diaper and the top edge of the back of the diaper are level with one another. They should be flush. That is how you know you have it positioned properly.
The bottom tabs are the first ones I fasten no matter how many tabs they have 4 or 6. Left side first loosely, then snug the right up nice and tight, then readjust the left to match the right. The tapes should be relatively equal in position on the front of the diaper.
Smooth the front out nice with your skin, and then attach the top tape tabs at some point on the front. Then match that on the other side. If it's loose, pull it tighter keeping the placements equal on both sides.
I always do middle tabs last, if they are there, and I always make these as tight as possible.
Now put on some regular clothes over your diaper and go do something else. Play games or read a book. Get your mind off the diaper for a while. If you are thinking about it you will simply still be turned on by it. Once your penis is flaccid, tuck it downward. Then readjust your tapes to snug up the space left behind.
After a while, if you get to my age and you are essentially wearing 24/7. It won't be an issue. Wearing and using your diapers will simply feel like wearing bulky underwear.