Study the tree's branches and overall shape to capture it in detail. Each tree has its own way of branching, so start by looking at its basic shapes and then filling it in from there. Once you've got the trunk and head of the tree outlined, fill in branches where you see them. Add in sky holes last—breaks in the leafy tree canopy where you can see the sky.
Think of leaves as masses, especially if you're studying the tree from afar. Don't outline every leaf; instead, add the most detail where leaves meet the sky, as that is where you can see their shape most clearly tunnel rush