DON'T take advice from a newbie to this forum!
Let me see, when cutting live trees, what is the desired outcome of the patch of trees you are working in? It can't be profit yet, or else the trees would be sold for lumber. What does the owner want?
A quickie lesson on thinning. There is thinning from below, which is what you do to keep the good trees growing. Cut the smaller, slower growing, deformed, or damaged trees. Be careful not to damage the leave trees--the ones left to grow to the final crop. Damage causes defect which will lessen the value of the tree when a commercial cut is done.
There is thinning from above, where you cut the tallest, biggest trees and hope the smaller ones will take off growing with the extra light and moisture available after the dominant trees are cut.
Then we get into thinning for a basal area volume, and that requires a bit more thinking, so I won't go into it.
Use wedges!! You'll have to do directional falling to minimize damage to the leave trees. Start on the edge, or next to an opening and aim your trees to that opening so they aren't falling through clumps of leave trees and hanging up. Then work your way into the area, aiming trees to the more open area.
It helps to get some flagging and figure out where your skidding trails (where you will be driving your tractor) will be so you can also fall trees towards that. More directional falling--more wedging, and less damage done during skidding.
I thought that the fallers here would hate thinnings. Nope. The good ones say they like thinning work. It takes more skill to get the trees on the ground without damaging the leave trees. It makes it more "interesting".
Time for a picture. This is my doing, I am not a faller, the stump is too high, but since it was a firewood tree, I gunned it towards the most open part, cut the stump high, and then cut the stump lower after falling. Most of the trees hung up as it was a tight place to fall trees, and they were limblocked. We pulled those down with a long chain and tractor.
This is a professional job.
During skidding--professional job.
This last photo makes me think of something else. In this part of the country, we risk losing more trees to blowdown for a few years after thinning, or until the crowns close up more. We usually luck out, but that's something else to consider.