You can find lots of ‘starting out’ posts using the search function and much of the advice given in the hundreds of posts on the topic is good; so I won’t repeat it here.
That said, I expect that you’ll need to be EXTRA, SUPER, MEGA careful with your UC in order to avoid flares. You are going to be intentionally stressing your body and digestive system, so you need to make sure you do your best to avoid ANY triggers you have.
For instance, you’re naturally going to avoid foods that ABSOLUTELY cause flares, but what about foods that ‘sometimes-but-not-always-maybe-possibly-occasionally’ trigger a flare? Are you prepared to avoid them altogether? Because you’ll need to.
Same with environmental factors. Stress, lack of sleep, hormonal issues, etc…etc…etc. All of these potential triggers can have an outsized effect on your digestive system when you’re fasting, so you need to make sure you take care of yourself properly.
Finally, I’d strongly recommend that you discuss this with your doctor. It’s possible that s/he can advise you on a medication change or schedule change that may make IF easier…or, at least, less prone to cause an IBD flare.