How do you do the "Lumberjack" skill?

the title says it all

Driving over fallen branches, tree stumps etc.

Just drive around and knock over the little trees, and you’ll run over the branches eventually. It’s pretty easy. I can’t remember exactly where the stump farm is at though. I think it’s north-ish.

Clearing near the Thicket drift zone has braches and tree stumps.

hitting tree stumps or some branches left on the ground. You don’t get it just from hitting normal trees.

I didn’t see anything actually relating to Horizon 4
On the map, look on the far left for Derwent Water. Go right some and you’ll see Lakehurst Forest. Go right some more and you’ll see Moorhead Wind Farm. Between Lakehurst Forest and Moorhead Wind Farm is a small offroad loop of road that is a small campground where the Drift Club story marker is. Scattered around the sides of the roads here are tree branches and a few stumps on the ground. Hitting one of those will get you the Lumberjack skill.

I drove around for a good while trying to think of where stumps or logs would be and had the worst time trying to find something. Hope this helps.