The best advice I can give you is to look around other towns for inspiration and then focus on a few specific things that you want. For example, let's say you want a nice little plaza with a statue fountain in the center. How many squares of free space would you need for that? Is there any particular area in your town that would fit it?
Once you have a pretty clear idea of what you want, you can place designs on the floor to mark where everything is going to be (like flowers, paths, trees, bushes or pwps) so that you can always keep an eye on the area even before you've completed it.
Once you have created one, two or at most three "nice" little areas, all that's left to do is tracing the paths that will connect everything and decorating them as you like. Trust me, an empty town always looks super-huge at the start and you'll probably have the impression you'll never possibly cover all that space, but once you really start landscaping a bit you'll realise you can't fit more than a couple "key" areas of interest in a single map, as the rest of the space has to be saved for villager houses and paths.
As for seasons... I know many people won't agree with this, but personally I mostly strive towards making a town look its best in one particular time of the year. And you have to decide pretty early-on which time of the year you want your town to be in when you record your dream adress at the spa.
Take a look at this guide
http://mischacrossing.com/post/84040749179/seasonal-tree-bush-grass-colors-guide and look at all the beautiful shades your town can possibly come in, depending on the time of year you set your game in. Is there any particular bush flower you absolutely want to feature? Then you'll most likely want to decorate your paths, fruit trees, flowers and even PWPs with colours that will match/contrast with it.
For example, let's say you want your town theme to be "red". You'd need apple trees (possibly perfect apples too), cherry trees, red brick bridges, lots of red flowers... and if you want to match all that glorious redness with red hibiscus bushes, you'd need to set your town in mind-to-late summer. Or maybe you like those delicious-looking red holly berries more? In that case, you'd have to plan for a winter town (before the snow comes). Or perhaps you'd rather see how all that red contrasts with bright yellow autumn leaves? Then set your town in October while you work on it. And maybe build a few yellow benches here and there too.
That was just an example of course, most of the fun comes from experimenting and see which flower colours and shapes look best together and how well they would fit with certain seasons or buildings. The best teacher when it comes to building towns is hands-on experience.
I wrote one hell of a wall of text here but really there is no secret to good landscaping that can't be summarised by what Shia Laboeuf himself said... Just... get out there and do it, y'know? XD
You might change your mind after a bit, regret where you build a PWP, miscalculate how much space you needed for this and that... it's not an easy process, but once you manage to make even a tiny little corner of your town look cool, the satisfaction will be worth it, trust me
