[EDIT] I’m in the middle of doing this. Once I got several ideas I used colored pencils and graph paper to map out my thoughts. I realized I needed prices so that was the first step. You might want to do the walkway first if you’re using slabs of stone which can get pricey. Plus, unless you can get the sand for underneath and spread it by yourself, you’ll need to hire someone. Flowers and trees could come next year. Hint: in the fall nurseries reduce prices on annuals, bushes, shrubs, trees and bulbs–great time to buy. I found nurseries are better than catalogs because the nursery’s plants are grown for your climate and water conditions.

[EDIT] BE aware your garden areas won’t begin to look like the pictures–they remove the old plants and literally cram in new plants fresh from the nursery when they’re doing a photo shoot. That’s why all the photos look so great. Be aware certain magazines cater to the West Coast which may not be suitable for plants in your area.
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