The technique of composition is to make the garden appear to be all one, single, unified design rather than a series of separate features which are unrelated to each other.
This would not be a good composition; the shapes and layout do not relate to each other:
These would be better examples of compositions using the same shapes:
Repeating shapes and overlapping them is a simple way to unify a composition.
When shapes and features in a design overlap it is called interlock.
Another technique is to repeat materials, so instead of having a wooden deck, stone path, brick wall and rendered house you use the same material in several places in the garden which ties the design together like in this example:
The decking and use of wood is on both sides of the garden, tying the design together.
Maintaining a similar planting style throughout a design also improves the composition, this can be done by sticking to a texture, such as large leaved jungle planting or spikey architectural planting:
Or sticking to a colour palette such as cool, or hot colours:
Mussklprozz via Wikimedia / CC BY-SA 3.0
Use a piece of blank paper, anything without lines or squares on it.
Draw a square on it, any size and in any position.
Draw more squares, some the same size, some different sizes. Some on their own, some overlapping e.g.
Choose a section from your drawing that you like and copy it (or trace it) onto another piece of paper – it doesn’t have to be perfect, a sketch is fine.
I like this section of my sketch:
So I would copy it out like this:
Feel free to move some of the squares if something about them bothers you.
I have lined up my two small squares with the edges of my big one and moved one over into the empty space and enlarged it a bit:
Now decide what these shapes could be if this were a garden.
This is very basic, but it is a great way to learn about combining shapes.
Repeat this process with circles.
Repeat this process with any other shapes you like, for example, ovals, rectangles, triangles.
Repeat this process with a mix of shapes, for example, squares and circles, or ovals and rectangles.