In reality a group of leaves arises from one of the arms of the tree. The groups lie in front and behind each other because a tree is not a flat thing. I’ll show you two ways how to draw them.
Exercise 1:
Draw each step before moving on to the next step.
Step 1: above the trunk you draw as lightly as possible circles right through each other. The circles don’t have to be perfect! Each circle represents one group of leaves.
Step 3: now choose in the other circles (groups of leaves) which are in front and which are behind. Each time erase the lines of the underlying circle.
Step 5: now draw leaves within each group. Let them point in different directions.
Exercise 2: DRAWING IN SHADES OF GRAY
First, very sketchily/roughly draw the entire shape of the tree in dark gray. Then, with a good eraser (or kneaded eraser), make light spots for the groups of leaves as in exercise 1. These do not have to be tight and neat. Then choose a light direction (in my example, side light from the right). Now start making more differences between dark and light, as loosely as possible. Between the groups of leaves, deeper in the tree and also on the shadow side, it will become darker anyway. Finally, add some details that allow you to recognize the shape of the leaves, not too regularly, not too precisely.
If you would like to draw a landscape with trees, exercise 2 is more suitable than exercise 1. Also where more trees/shrubs overlap each other, you will benefit more from distinguishing gray values.
DIFFERENT TREE SPECIES
In exercise 1 I started with circles as basic shapes but different types of trees may of course have other basic shapes. Later you can make those basic shapes leaf-like again. In this way you can even make a real Christmas tree (the bottom two).
EXTRA 2: copy as lightly as possible one or more of the following tree models. Look at photos of the chosen tree species or the tree in real life, even more beautiful of course. For example, in a weeping willow the leaves point downwards like tears. You can also make the drawn leaf groups gray immediately and then continue with light and shadow as in EXTRA 1.
Christmas tree (start with the two groups in the middle just above the trunk, zigzagging from bottom to top)