Congratulations. I could never get the hang of it - though I was pretty impatient at the time. Might be worth revisiting some day. Let us know how it goes!
Thinning is the secret to success. The GW paints are formulated to be brushed on, so they will have to be extremely thin, requiring multiple coats. That's not necessarily bad, as multiple thin coats give better coverage.
What is important is the consistency of the paint. Some 'experts' say to thin the paint 30%, some 50%, some say use water, some say Windex, others alcohol. All paints are different, metallic, to dark colors, to lighter colors, and using a percentage to thin is useless. Thin each individual paint to the consistency of milk, and you should be good. Also, use the thinner recommended by the paint manufacturer. In many cases that will be water, but use what is recommended.
How big is the base of that marker? If it's as small as I think, you already have a delicate touch. I look forward to your progression.
Even using an airbrush, finishing up with a drybrush is a good idea.