firstly, you have to clear everything out to make it look nice. it's the only way to have a blank canvas for your town. then, plot reset your villagers into places that are convenient whenever an old one moves out and another moves in (sometimes, you will have to let go dreamie villagers in bad places, which sucks but is necessary)
after that, plan out your pwp positions and pathways from main buildings such as your house, your plaza and retail. after that, plan the trees and bushes, with little patterns with words on to identify where each thing is supposed to be eventually.
it's tough to visualise in your brain, but go to dream towns and take inspiration with little bush/tree combinations or flower combinations that look good to you. paths, unless you're going for a natural look, should be fairly straight and simplistic, which not too many neon colours, as you want flowers to make the paths 'pop', with the colours being brought out through them.
practice makes perfect, of course! if you don't like a set of hibiscus bushes in a line, perhaps alternate between trees and bushes, or mix things up with a zig zag line!
for flower combinations, if you let your town flow through the seasons naturally, you will have to change them every few months to a new set and move the old ones to the beach, or have a bright set that is very fitting all year round (black and white, for example). if you don't, you can pick flowers that pick out the colours of the season - pinks for spring, oranges for autumn, reds and yellows for summer, for example. for the combinations themselves, it's all about experimentation. i like a more natural, mixed look personally, with a small amount of tonal contrast between the flowers, but a focus on colour rather than breeds of flowers.
really, to make a great town, it's all about dedication to making your town how you like it, then presenting it with confidence. good luck!