Jess said she put underglaze in all the veins that that the fern indented onto the cup and wiped it away. My question is doesnt it soak into the bisk and if not how hard is it to "wipe off" the rest of the cup? Also do you use a brush or an applicator?

Underglaze tends to sit on the surface of the bisque and can be wiped away easily. I personally use a brush for filling details with underglaze.