Okay, it's not the best idea for a baby bottle. This requires a bottle like... a dasani water bottle or other water bottle. If you're in a situation where you can't use a real baby bottle, this might be a good disguise from the real thing.
Anyway, what you do is take a tack and poke a hole into the bottle cap (or several near the original hole depending on how big you want it to be). Then you fill it with whatever you want and drink from the cover.
I do this all the time. I actually learned it from a friend of mine in school. They weren't an ABDL though. It does come out slowly, depending the amount of holes you put in.
I hope you like this idea. ^w^