Everyone's flare is different, some have D, some have C, some have bleeding, some don't. There is not one answer that fits all so you have to describe your symptoms and then people can respond.
"In general" probiotics are good but sometimes you have to try a couple different to get one that helps and doesn't cause unwanted symptoms (gas, bloating).
"In general" fiber is good and many people like Metamucil but for some fiber can cause problems.
Diet is a very broad field and not answerable by "What can I eat?". Start keeping a food journal and noticing what your gut does. Sometimes it's not immediate, might be the next day. Sometimes you think "Aha, my gut quieted down, that's good" only to find out that whatever you ate caused massive constipation (C). It's trial and error.
Good luck, it's worth it but hard work being your own detective.
In remission April 2010 after 10 years of UC with no remission ever
Jan 2010 began SpecCarbDiet (modified to remove dairy, fruit & juices,)
Gluten-free=bleeding stopped in 3 days, 95% remission in 5.5 months on SCD
SCDiet: Gluten/grain-free, starch-free, low carb, honey as sweetener
(9)Balsalazide+(6)Citrucel, mesalamine enema as needed
Spinach & sunflower seeds daily, (1) VSL#3 nightly