A garden should contain things you like, within reason, where you want to see them, as it is you who is going to have to live with them. There are only two fundamental planning considerations to bear in mind: first, the plants should be suitable for the area you want to put them in; and second, big plants should not be placed in front of small ones. All other aspects of planning are purely personal ones: what suits one persons tastes or needs may be an offence to another. You might like borders with crisp, straight lines and plants placed like rows of soldiers. Your next-door neighbour may tell you this is wrong — borders should have wavy informal edges with what they contain allowed to naturalize into an informal jungle. But this is his taste; if intricate geometric designs full of bedding plants please you, then providing you have the time and money to maintain them, have them.
What is important is to get your ideas and observations into some sort of order. The best way is to start with a scale plan. Obtain a large sheet of graph paper and decide on a scale which will enable you to draw the whole of the part you are designing (back, front or whatever) comfortably on it.
The next step is to go outside and measure up boundaries, with a check on diagonal measurements, positions of existing features, trees, shrubs, beds, walls, paths, patios, sheds, clothes lines, etc. that you intend to keep, and transfer these accurately to the plan. You should also indicate which way the garden is orientated. (North can be determined using a compass, but another way — if it is the right time of year — is to see where the sun rises and sets on 21 March or 21 September. On these dates it will rise due east and set due west.) This will affect the areas of shade in it. For example, if your property is at the southern end of the garden, it will exclude a lot of midday sun and so the house and therefore the garden is said to be north facing. It is well to know these things at an early stage in designing as a north-facing part will be cool and shady, a south-facing one hot and sunny, a west-facing area will have hot afternoon sun in summer, and be sheltered from cold, easterly wind in winter and an east-facing aspect will receive cold winds and early morning sun which can be harmful to buds and new shoots during frosty weather. You can then choose suitable plants for each type of situation.
Indicate on your plan any areas which receive an appreciable amount of shade from high walls, big trees, buildings, etc. and show the extent of this shade if such features obscure the sun for a fairly long period of the day. It will be useful later when you come to deciding what you can have in a particular area.
The next thing is to consolidate your wants, needs, likes and dislikes by preparing some sort of questionnaire. This all sounds rather unnecessary, but it does save you making a lot of mistakes in the long run. Ask yourself some simple questions, give honest answers, and you are well on the way to planning your patch the way you want it.
what can I afford? Try to have some sort of target figure in mind when embarking on a planning scheme. Even if you are going to do every bit of the work yourself, trees, shrubs, slabs, fencing materials, grass seed, all run away with the spare cash. If you are young, of course, chances are that you will be able to spread the cost over several earning years, so there is nothing to stop you from aiming at something you cannot afford all at once. However, if this is the way you intend to go about it, do try to plan the work so that every stage is an entity. Nothing is worse than a job that looks as though you have run out of money. For example, if cash is really short, plan the beds, borders, lawns, paths, etc. in advance, then tackle each job and finish it as funds allow.