we went on a 1st aid course last weekend - 3 hours - which covered choking vs gagging. Would definitely recommend you do it. Nothing like hands-on practice with a special 'baby' and opportunity to ask questions.
Your baby will gag whatever they put in their mouth - food, fingers, toys, spoon, etc. They soon learn how far they can put things and they'll cough to move it to the front again.
The foods our trainer warned us about were grapes and cherry tomatoes. Crush them first.
But the major one is chopped up sausage. Scary but several babies have died from discs of sausage getting stuck in the windpipe - so cut them lengthways too.
Even adults choke and we covered that in our course too.
We let our son play with a spoon before we started BLW and he would gag on it, but obviously couldn't choke! I think that helped him learn that when we handed him spoonfuls of porridge, for example, how far he could put the spoon in.
It will be messy and food will get thrown away but it's fun too, it means you can soon eat with your baby in a cafe rather than have to feed them, it's good for fine motor skills and supposedly helps with language - all that tongue and mouth action!
We use leftovers of our food e.g. spaghetti in cheese sauce with leftover veg mashed into the sauce.
Watch out for bread though - it can get wadded up and stuck in the roof of their mouth, as can peel of mangos or courgette. We do a fair bit of fishing out with our finger!