Once you have surveyed your property and have an idea what it might look like put that idea down on paper. Just a rough sketch will do but put as much detail into it as you can think of. Remember that plants grow up so if you are planning on trees or bushes consider their final size in your plan.
Now that you have it on paper take that drawing out in the yard with you and walk around to see if things will fit and look the way you want. Change your sketch to reflect changes that you want to make.
Next comes the fun part. Make a model of your project. Don't choke, it's really not that difficult and it doesn't have to be perfect. It will just give you an idea if you plan will really work. Here is the model that we came up with for our back yard.
We used modeler's clay from the hobby store on a piece of plywood to give us the general lay of the land. I built the back of the house with our decks sticking out from plywood to give it some scale. That's what you can sort of see on the right side of the picture. We knew we were going to have a storage shed with a water wheel on it. We made that out of a block of wood and just cut the roof angles on the table saw.
We then added the details like paths, river, pond, grass, etc. We used what ever was handy to give it some realism. Coffee Grounds for the river and pond, Easter basket grass for the lawn and what ever else we could find for the rest. Use your imagination, it's really fun.
Now get started on your model and have a ball. One more thing, let your grass and weeds grow. Don't mow them or weed whack them we'll be using them in the next step of this project.