We waited until 6 months simply because usually the earliest when it's recommended to start, but beyond that just how well they can sit up in a high chair or wherever you are feeding without slouching to the side or back or forward. I know some people start weaning early using a bouncer or other reclining seat (that's probably not even safe then), but I would say it's definitely not safe for offering finger foods if baby is lying back. Basically, you want your LO to be able to sit in a position so that gravity won't work against them. If they are sitting up pretty straight, then they will have more safe control over the food in their mouths. If they are reclining back or slouching to the side, it's a lot harder to control food in the mouth, and especially when lying back, risks food slipping down the throat before they can gag and spit it out. So for me, just sitting well was the most important thing.
At 6 months, some babies really can't easily bring food to the mouth yet, but that's a skill that's developing through eating, not really a prerequisite for offering finger foods. If your LO is older, they may still not get it at first, but just time to play with food and practice will mean they get it eventually.
That's really it. It's much more a matter of YOU being ready. If you're nervous about it and hover and panic the second anything goes in, it will make it stressful for your LO. Not how you want learning to enjoy food to go. So you really do need to sit on your hands and be slow to intervene unless your LO is really choking (learn the difference between choking and gagging, because most 'choking' is really gagging and you don't really want to intervene then because it's your LO's way of preventing choking). I have friends who wanted to do BLW from the start but the dad is very, very nervous about food. Every single time their LO ate, she supposedly 'choked' on something (she didn't, I was right there next to them). But he was constantly banging on her back and literally dangling her upside down trying to get food out in a panic. She was fine. But it was him who wasn't. As a result, at 2, she's now a really nervous eater and won't try anything but a few very bland basic foods. I think a lot had to do with how much anxiety there was about mealtimes early on.
So really, just see it as exploration and fun and sit back and try to relax about it all. It's really fun. My daughter is 2 now, she's never had a puree, finger foods from 6 months and she's never choked on food once. Her first full meal was roasted carrots and parsnip sticks, mashed potato, and a yorkshire pudding, but she also had lots of cucumber sticks, watermelon sticks, and toast fingers with mashed banana or mashed avocado that first week.