I built my first beds out of the rafters of my neighbor’s garage when he took it down, and when those rotted after several years I replaced them with treated lumber treated ( treated might not be a good idea) and in a corner near a fence I used field stone. We put heavy landscaping fabric between the beds and laid down a layer of pea gravel for walking. It’s best if you can have the bottom of the planks and inch or so below grade to prevent erosion.
Later I added the stock panels to keep deer out.
Don’t use lag bolts to fasten the long planks to the cross pieces- they’ll eventually work out. 16 penny nails actually work better. Wouldn’t hurt to fill the joint with construction adhesive to help keep water out as much as to bind them, I suppose, but we didn’t and they’ve held up for maybe 20 years or so now.
We kept them to 4 feet wide, originally expecting to work the beds from either side. When I added the fencing (stock panels) we still found that you can manage OK from one side. Sometimes I lay a plank down to help.