A garden plan can really be useful if the garden you have acquired is new or has very little growing in it.
Exciting as it is, whenever you acquire a new place to live it is always best to leave the garden alone, apart from general weed and lawn maintenance, for at least the first season. If the garden is with a new build you could do a plan straight away as there is nothing likely to grow.
Leaving the garden will enable you to see what bulbs emerge from the ground, what shrubs or trees blossom and any other flowers bursting into bloom. These could be recorded on paper their location and time of year the blooms appeared. This will also help with your planting scheme.
Take a paper and pencil and draw a square, rectangle or a shape that can represent your garden. Looking out towards the garden from the house. Start with the view that will be seen the most or take a look from two major windows. It is nice to have a slightly different angled view on a garden.
Depending on the position of the rooms within the house one view would be through a back living room patio door or when seated in a conservatory. The other view could be from a study or kitchen. Remember that the front view can be as important as the back regardless of the size of space available.
When looking out at the view where does your eye rest, is it on a existing tree, looking along a garden path, at the corner of a fence or a shed.
On your paper mark the objects that are existing such as a tree or anything else you do not want to alter.
In one of the gardens I have had there were a few existing concrete paths that I had no intention of changing or moving. They led to different parts of the garden with the main one heading up to the shed.
Do this step over a few days and at different times during the day. On your piece of paper mark where the sun shines on your garden with the time it is there.
This will give an idea of the shaded and sunny areas of your plot. A really sunny corner wouldn't been a great place to put a shed but it would be ideal for a seating area or a second patio if the garden was north facing. A shaded part could be a children's play area or a place to put a shed.
A lot of plants thrive in mottled sunlight rather than full sun all day. There are also shade loving plants if you find the sun doesn't visit very often. This will then give ideas of where to place structures and flower beds.
The garden may already be established and works well for your needs. Apart from planting some annual or perennial flowers that you like there could be no need to change at all.
The garden plans above were already in place when I moved into these houses. They had all the elements I wanted and apart from adding some flowers in pots and borders, these gardens worked so I didn't change them at all.
Hope you have fun working out your garden plan.