She started with priming the plastic using Mr. Surfacer spray primer and then let it dry completely. She took her chosen color (Tamiya acrylic flat blue) and diluted with plain water. She only used a small amount for the first try (2 drops of water). She then put a small amount on the sponge and dabbed it on - and then let the part dry completely.

She wasn't happy with the paint coverage so she diluted the paint again with 4 times the amount of water she used previously (8 drops?) - but she didn't say how much paint she started with so use your own judgement. She then used the more diluted paint for further coats until she got enough coverage.

So: (1) spray the part with primer, (2) dilute your chosen acrylic paint with water, (3) dilute with more water until you get a thin paint that goes on easily, (4) don't worry about the bubbles, they are small and will mostly go away while drying, (5) apply several thin coats until you achieve coverage.

Try the approach and experiment a bit. I find a lot of Japanese modelers just try new things and record a video on how it works out.