Learning how to draw leaves on a tree is not as difficult as it sounds. But it does take some practice and time. Fortunately, you can find plenty of leaf illustrations online and in books to help you get started.
You'll start by drawing two circles on top of your tree, which will act as the trunk of the tree. Next, draw the bark of your tree with some curved lines.
Sketching a rough outline will give you a base for adding detail later on. Don't worry about making your tree perfectly symmetrical; just strive for balance and a natural look.
If you're familiar with the common structures of leaves, such as the tip, margin, petiole or stalk, lamina, midrib and veins, you'll be able to simplify the drawing process and create more realistic looking trees.
The main thing to remember is that no two leaves are exactly the same, so use your reference photos or leaf illustration as a guide for your drawings.
Once you have the basic tree outline completed, begin adding detail to your drawing by tracing over it using a pen or brush. A 0.5 or 0.7 mm pen will work for the outline, while a 0.1 or 0.2 mm tip works better for the details.
When you're ready to add color, choose a soft pencil or a water-soluble crayon. Then shade the tree so that one side is lighter than the other, based on where the light source is coming from. This will help your tree to look like the sun is shining on it.