I always use something pretty small. I use a dixie cup or a shot glass, but I have seen others use a paper cup (12 oz or something around there) and cut it in half so it’s not so high. Just make sure whatever you decide to use, you don’t mind it getting ruined. It should be about an inch or 2 in diameter.
As for the polish that can be a bit of a hit or miss. I have found that most Sally Hansen Xtreme Wear polishes work and they are only $2-3. Wet n Wild also have worked for me in the past and are also around $2. That way you aren’t wasting your expensive polish.
Glitter polishes have never worked for me. :( They immediately dry in the water and are impossible to make designs with.
I hope this helps! My only other tip would be to work quickly while adding the drops of polish. Only leave about 15 seconds between drops or it may dry too quickly. I think that’s a good time estimate.. I’ve never actually timed it so you may have a little bit longer but not much. lol
Hope it works for you!!