From all gardening programs and my own experiences I've found this to be the best course of action:
1) "Meten is weten" is what we say so MEASURE, MEASURE and MEASURE your property(boundaries), all existing structures and put it on paper.
2) determine what to keep and what's to go (buildings, fences, trees...)
3) make a list of what you want FROM and IN your garden (trees, walls, hedges, raised beds, shade, sun, terrace, water feature/pond....) and give them a rating
4) BOUNDARIES! they'll take up some space (hedges more than a picket fence) I see so many people starting flower beds etc and then destroy them later on because a fence/wall/hedge still needs to go in. You're better off working from the outside in. The sooner you decide on this the better I think.
5) think about what you acutally CAN have (limited space usually is the deal breaker)
6) put it out on paper. it's best to take your time and think about it. you're propably going to change it multiples times until you're satiesfied
7) lay it all out using spray paint, sticks and twine....anything to visualize it IRL. Now you can still adjust sizes
8) start with construction of any buildings and hard surfaces (terrace, deck, hard stone paths...) and digging (pond)
9) finally PLANTING: a) if you're going for (a) hedge boundary(ies), put those in first
b) (large) trees. it's easier to plant around them than trying to tip toe through
your lovely flower, never mind digging a big hole without uprooting and
d) (taller) perennials
e) groun cover
Ofcourse do keep in mind: soil, water, temperature and light. These ultimately determine your plant choice