My process is similar to Su Ba's. I like to dig down 2+ feet & put a layer of wood on the bottom and then make layers out of whatever I have on hand (usually rabbit manure, weeds, leaves, junk mail, etc) until it's slightly above ground level.
In my
experience & climate the wood seems to be the key component to holding moisture. In the sunken beds where I've just used
cardboard and/or paper, instead of wood, I found it, either, broke down too fast to help for the whole growing season, or it didn't hold
enough moisture to meet the water needs of the plants. Even if I don't have logs/limbs available, I still gather sticks, bark, and/or wood chips to give it a woody base. I'll also use small sticks/bark/chips as one or two additional layers of the bed
Once it starts to break down, it settles back to ground level or a little lower. If it sinks too much I just add some soil to the top to bring the surface level back up.