Pretty basic, a square frame with 2x6s, this one is 8x8. We used 2x4s to cover it just because they were cheaper, but generally one would use 5/4 x 6s. Use something to space the boards evenly, I used the flat end of a carpenters pencil.
Step 2: Placement
I used flat garden stones under each corner to make it level.
That's about it, it works well, and is quick and fairly inexpensive.
Thanks for reading.