Lots of good advice up there ^^.
My first 3 have all done it a bit younger, so feel free to ignore this, but I've used a long straight stretch of tarmac lane for the final step. We ride out there with stabilisers, and then just at the point where the lane starts to slope gently downhill, I take them off and run along beside with a hand on a shoulder. I wait until they are confident enough like that that I'm not doing any of the balancing and then take my hand off briefly, and then again for longer, and so on. The downhill helps because they keep going. If need be, I go back to it several days on the trot before I start letting go. Lots of variation from child to child.
The first one went straight from stabilisers to that. The other two had an intermediate stage of playing "the whooshing game". No pedals, lowish seat, push them progressively further between mum and dad, with enthusiastic whooshing noises and loud cheering. Again, this is on tarmac in the cul-de-sac because the local grassy areas are a bit too long and bog them down.
The sound effects are important because you have to convince them that "THIS IS FUN". i would guess that different sound effects would work better for a 6YO. "Wow, you look just like a ballerina" maybe. "It's your magic pony"!
If they do fall over, I suggest not making much of it. A nonchalant "upsy-daisy" says it's OK to fall off.
It's lurverly when they go solo though. Tears to the eyes, literally! But you do need to take your time and not rush them.
Good luck
J