I have some bsfl , how do I turn them into the fly? Just wait?
Pretty much lol.
There is a trick with the Green/Blue Bottles that I have not tried with BSFL, but might be worth a shot.
With the GB/BB when you fridge them they kind of suspend. So people allow them to age right up to pupation, till a few in the cup start to emerge, then they fridge.
That way when the large are pulled from the fridge, they will usually emerge shortly after.
You can buy already ready to emerge soon BSFL they are called Crawl outs. In my experience with my last time buying those specifically, it didn't go so well.
It took a long time for my other BSFL to become flies, but they did so at different times. So Everytime I opened the container of 500, there was 5-10 flies, every other day. With the crawl outs, the majority hatched in a week, and I lost hundreds of flies, more were emerging then I could feed, and so I had to let them die

.
It since then slowed down to 5-10 a day, but I lost most of my flies, so I will be trying the fridge technique next time, as my current panther stopped eating the larvae and refuses it, but loves the flies.
Also as Carlton said. The regulation in the US which IMO is a tad harsh, makes it very hard to find acceptable feeders outside of the norms as well.
Places like the EU, and it would seem Canada on some, are much more lax. Some of the USDA bans are downright silly. Hissers were against the rules, for the longest though not a plant pest. Now normal Hissers are fine, but Halloween Hissers are technically not legal, they are not plant pests, they eat leaf litter, doesn't matter in the US they seem to implore a ban first ask questions later policy sadly.