I've tried to learn from various online tools as well as some books on it. The online resources are good, but the books have largely been older (and some new) rendering books that focus on design and not the illustrative artwork I've seen by many artists. Thankfully, I've found a few instruction books that give some more illustrative advice, such as iii Academy (which just published the first collection of lessons) and the Intro to Coloring with Copic Markers, both from the North American distributor, Imagination International. Those resources helped me get a better grasp of the basics and to go from there.
But I still have to practice it, hence a challenge: draw a simple marker illustration every day. I've done pretty good so far, having finished my first week.
I sometimes start with a thumbnail sketch in my planning sketchbook, and sometimes I go without a clear plan. I have been using references for ideas, planning, and colors. I then pencil in the sketch on Strathmore artist tiles, currently the sketch artist tiles. They aren't the greatest paper for markers, but I chose them mostly for the convenient size; at 6x6 in., they aren't so large as to be so intimidating that I can't put something down or finish easily, but not so small that I can't get detail in. I then ink with the various marker pens I have before going into inks.
If you wish to follow on a daily basis, you can follow me on Instagram.