Good question.
Myself, I prime the included microfiber pad with two sprays of the product then apply in straight lines. I generally just rub back and forward with straight lines because on a light colour it is very hard to see where you have applied it. Some people use circular
motions but it's really up to preference, there is no correct "motion" to apply it (although I'm sure there will be someone who will disagree with me). I will generally spritz the pad once for every door I do, or maybe once for the fender and a little bit of the hood. For me I generally use between 2-3 sprays when I'm applying on the hood, with there always being enough left on the applicator to apply some partially to a fender before re- spritzing the pad.
You will see it becomes thicker of you were to go over the same area twice, but that doesn't mean it will be much easier to tell. I usually look on an angle like you are trying to look for swirls (the Brinkmann swirlfinder light would possibly be useful here to check coverage of the product). I also swipe my finger on certain areas to check that the applicator is depositing product as the paint will be incredibly slick while it is wet. One thing with is that they mention on the side of the bottle to not touch it for an hour afterwards, so no fingers or anything. I generally re-wipe the area I've touched, with the applicator just to be safe.
Hole this helps, if you have any other questions feel free to ask.
Cheers,
Matt
Sent from my iPhone using Tapatalk so please excuse any grammatical errors