It is a red shop with yellow trim. Its sign consists of a striped pink donut with a wooden plank on its side that reads "Flipper's Donuts" in pink lettering.
In the episode "Patrick-Man!," Patrick can be seen rummaging in a dumpster behind the shop. He finds a purple frosted donut in the dumpster and eats it.
Trivia
It is one of at least two donut shops in Bikini Bottom. The other is Donuts from the second movie.