As requested by several folks, this is a picture How-to-do-it for my latest Seriously Raised Garden.
These materials - the Cedar Raised Bed kit came from amazon - as did the Wholly Cow poop bag of fertilizer. All other materials were acquired locally.
The kit came with 21 4 ft cedar slats, dove tailed at the ends to fit in the corner posts, already grooved, as you can see. Rather than build this as a 4'x8' raised bed on the ground - since I have serious trouble getting DOWN that far to plant, weed, or harvest, I build my garden areas up on concrete blocks. This is workable for me and it also seems to help with weed control. However, it requires daily watering. And I find a 4'x4' size more manageable, so that gave me the left over parts, like slats, for the bottom.
There are salt-treated 4x4s on the concrete blocks - the cedar frame fits on top, the extra pieces of siding are nailed to the 4x4s leaving a drainage space. Do NOT use treated wood next to your soil - the chemicals can leach into the dirt and be absorbed by your plants.
To keep the soil in the garden and not let it wash out, we put a cover of "rat-wire" over the cedar and a double cover of heavy garden cloth (that roll of black stuff) over that, stapled to the sides.