Minecraft wiki entry for Ultra_hardcore_survival, associated text: A great way to check out if there are any players in the vicinity is to trap yourself in a small capsule. The lack of lighting causes everything to be dark except for name tags, which are now more visible than regularly.
Gold and apples are going to be your top priorities. Apples can be found on the surface and dropped from oak leaves, like saplings. In many cases you'll have more apples than gold.
While waiting for your items to smelt, consider farming gravel to gain enough flint to craft arrows.
With the addition of suspicious stew to Minecraft, regeneration is much easier to get. Considering how mushroom biomes remain one of the safest environments, and the ease of growing flowers, suspicious stew may serve as an excellent early game healing item, and a much cheaper alternative to potions, albeit not nearly as effective.
Consider checking out YouTubers or Twitch streamers to see their UHC strategies.
The game usually goes up to 1-2 hours, so make sure you won't starve (in real life) or go to restroom. Keep some water beside you and snack if necessary.
Check the rules before going into a game of UHC. Some may have player head apple and strength/regen disabled, so you might just want to read it before joining.
Strength is extremely beneficial (if not disabled), being able to deal an extra 3 damage in Java Edition, and more damage in Bedrock Edition on only the first tier. Speed is another potion that is handy, being able to go 20% or 40% faster.
NEVER AFK! You don't want to come back and realize you got killed.