Place a blank sheet of paper beside a picture of your garden, and the point becomes apparent immediately. Your picture presents all at once fence lines, pots, furniture, weeds, paving, shadowing, doorways, windows and so forth. A rough base sketch reduces some of this clutter. The base sketch provides a map for you to use to work out where you want pathways, plantings, seating and focal points, rather than trying to find all of these things all at once in your actual outdoor living space.
Your base sketch does not have to be pretty. It does not need to be accurate either; it just needs to include the existing and unmovable items on your site such as walls, fences, steps, existing trees, doorways, windows, drainage lines, paved areas, sheds and so forth that do not move around. Use graph paper if you want to work to scale, or just a piece of paper if your outdoor living space is small. You will only need a tape measure, pencil, eraser and a few photographs of your site in order to create the first base sketch. It is not a plan for your new garden yet, it is just somewhere to start working out your ideas so that you are not wasting money or getting tired doing this work in situ.
If you are a beginner, a base sketch can really help you. Often in your head you think you know exactly what to do but then once you get out into your garden the plan becomes crowded. A planting bed that looks large enough when measured in your head is not large enough when you consider how big a shrub in that bed will be when it has matured. A pathway that looks obvious in your mind might block up one of your favorite sitting areas. A focal point plant that looks great in a garden center or in a picture online will not necessarily work well where there is an existing tree, window, or already established border that draws the eye in that same direction. By putting these on a piece of paper the issues may become much clearer since the whole garden is right in front of you.
If you want to create a base sketch, try working on it in a series of steps. Firstly draw in the outline of your whole space, then add your fixed, existing structures. Then next make a note of the main access points to and from the outside space, for example a backdoor, gate, path to a garage, and a patio stairs. After that, lightly draw out the main paths people will walk along, where people will sit, and where the views will be. At this point you can try to mark out different planting areas or other outdoor zones. You could sketch out a seating area, a planting border and another open area for pots. You are now using this to create a plan which is related to how you will actually use the space, not just making a pretty picture.
Tracing paper is useful for doing this, because you can lay it on top of your garden base sketch to see if one version is better than another, without having to start again. On top of this, draw a curved border then a straighter one in places, move a path a little way along, put the focal point a few feet further up the garden, make a planting border wider and then make it smaller again because you need to get back there with your mower. As you do this you learn how one garden area affects another, how a path has an impact on a planting border, a seating area affects what you see from it, a group of shrubs changes the shade and sight lines through the space, and so forth.
It is easier to plan plantings when you have done this kind of sketch, since it is easy to plan the role of each planting area. If you have a border on your plan that runs parallel with the main path, for example you know that you will want low edging plants nearest the path, mid height plants in the middle of the bed and taller plants in the back near the fence. In another area of your plan you may know that you only want a few plants in that bed since you want to be able to move easily between beds to water and prune them, or to carry out gardening tools through this space. The point is to learn to see a blank area of your plan as a good thing, that is necessary to make your outdoor living space actually work!
You know that your plan is good when it answers some of these common questions. What are the fixed and existing features? How does a person get in and about the space? Are certain parts of the space sunnier than others, smaller than others, damp or dry, in an open exposed situation, or are they awkward to get to? Where would a planting border be able to fit on there without blocking access and views? This sketch has done its job already even if it does look very rough. The next landscape design you draw will stand on much firmer footing than the first!