So I finally had a game where I was actually able to craft things. I started trying after this thread. The biggest barrier is food - if you don't know Seges, it can be hard to keep yourself fed while hammering out items... and even then, you might get hungry faster than your prayer timer. The second barrier is tools. While you're guaranteed to find a forge in the UT, you are NOT guaranteed to find a hammer. I had to make due with a broken warhammer I found. Materials, I've found, are no trouble at all - in fact, disassembling unwanted items as you go is a convenient way to keep them out of monster hands. You can then leave the lumps and sticks wherever you want, and monsters can't put them to use; this keeps them from cluttering up your inventory while making sure they're not a danger to you later.
So anyways, here's what I've learned.
1) Hammering on the anvil causes tiny explosions. I recalled some time past that this would occasionally destroy the anvil - I don't know if this is still the case, though, and it didn't happen to me.
2) These explosions can - and will - damage and light on fire anything nearby. Don't leave books or scrolls on the floor by the anvil while you're working. (Unclear if they'll light things in your inventory.) They WILL break walls.
3) Using broken tools slows crafting. You are warned of this when beginning. Not sure how much that impacts your work. I'm also not sure if using a warhammer makes any difference, as opposed to a normal hammer.
4) Oh yeah, you need tools. You need a hammer to work metal on the anvil, you need a knife to extract poison... and possibly to work cloth-like materials.
5) Disassembly sometimes requires tools. Some items, like axes and polearms, can be dismantled with your bare hands. Others, like maces, require a knife, and still others like swords must be done at the forge.1
6) I'm not sure if this is because of using a broken tool, but every item I produced (two copper gauntlets, two fairy steel beast claws, an iron full helm) was finished in a "broken" state.
7) Crafting does not (currently?) include a "repair" option.
8 ) It doesn't seem very clear how many ingots of material are required to create an item. This is complicated by the fact that ingots of different sizes will stack together.
"Put more stuff in the... thing where... more stuff goes in."