I've had the same town for 4 years now, and I've done a lot of landscaping. I've changed my town's entire look over and over again even though I thought I had a good layout figured out. Even if you do plan it all out, you'll probably change your mind so don't get too hung up on one particular look.
That being said, I like to start with making a path/ walkway leading to all the important spots in my town. Having a path, even if you don't use an actual design, is nice because it keeps you from creating unsightly bare spots all around town. I like to line my walkway with trees, rare stumps (which takes a long time), or in Glendale, with bamboo fences.
Once the path is lined, I add pwps that help to improve my town's status. Once a considerable amount of space is taken up by pwps, and I've figured out where I want to plot reset all of my villagers (and you should definitely plot reset), I fill in space with fruit orchards, bushes and rare flowers. Sea shells, coral and mushrooms are also nice space fillers.
If you'd like, you can visit Glendale (which isn't quite finished yet), my main and oldest town, in the dream suite. My DA is in my signature :3