1. Landscape analysis. How does the drainage work? What elements that are already there do you want to keep (a tree)? Where does the sun shine...and where is it shady? Do a sketch map of problems and opportunities based on what is already there.
2. Landscape program. What do you want to do in your landscape? Garden? Storage of cars and other toys? Do you have animals? Do you play games? Do you have kids who need to play? Do you want to entertain, like parties or barbeques? Where do you want shade and where do you want it to be sunny all the time (food garden for example)? Are some areas multi-use and others single-use?
...What do you NOT want to do in your landscape? How much upkeep do you figure on doing? Are you into lawns with all the expense and time needed to keep them up? Using a different color of ink or pencil from step 1, draw in things you want to do and where you think they would work best, given the problems and opportunities in step 1.
3. Outdoor "rooms." Once you have an idea of where your activity areas will be, based on step 2, think of the yard as an extension of living space OUTside your house. Think about how the different areas as a series of related outdoor "rooms" that are defined by hardscape (bricks, decks, walls, driveways, walks) and softscape (trees, hedges, plant beds and borders). This is also the time to think about how you will water and light your yard, if you haven't already done so.
4. THEN and only then, do you decide what plants and trees to buy, as well as lawn installation location, building of walls and decks, etc. If you put things in before you do analysis or have a plan, it can be expensive. You may plant a cute little tree in a tight space where it "looks right" but where in 10-15 years it will crowd your house or cast shadows over a food garden area, or grow into power lines.
You don't have to install things all at once, depending on time and budget. You can phase things in. But a good plan that you stick to will keep you on track, will measure your success, and will end up as a unified and beautiful design.