You have interest in this kink, maybe you thought about it before, but are unsure how to start? Here's a few points, and things you can do, solo or with a partner (make sure they fully consent, of course):

- Start small: at first you should start with short amounts of time (as short as ten minutes), learn to listen to your body, how it feels, and how it arouses you. Maybe do some foreplay, or read a kinky story during that time. You should also start with denying the possibility of peeing, not of defecating.

- Resist the urge to hold for extended periods: it is tempting to push it to the extreme right-away. Don't. Learn about your body and go slow; you could injure yourself otherwise.

- Aftercare: after getting relief, you should rest and take care of yourself (and your partner, if applicable) for awhile.

- KISS: again, much like length, keep it as simple as possible. No paraphernalia, diapers, plugs, etc. Focus on the urge and yourself.

Practical example: a quiet afternoon, start by drinking a good cup of water (around 300ml) and wait for the urge to come; then either ask permission to your partner, or put a 15min timer. Relax for awhile, enjoy yourself - maybe masturbate (don't orgasm until the timer is done...unless you are into that, oc). Once the time has ended, you should let yourself climax if you were close, and go relieve yourself. Then a bit of aftercare.

If you enjoyed it, rinse and repeat a couple days later!

Hope this helped; I'm open to any and all question, provided they don't pertain to personal information (including asking for nudes).