Gather supplies. Wagon optional.
We're gonna drill pilot holes, so you don't split your boards when you screw them together. Measure 3/4" from the sides and end of your 8 foot boards.
Pilot holes marked.
Drill your pilot holes. Try and make sure they go through straight, perpendicular to the top of the board. Both ends of all the 2x6x8 should have two pilot holes.
Time to build the first layer. Line up your boards...
Make sure the tops align...
Screw through the pilot holes to build your base level frame.
Time to place it exactly where you want it to be (most sun possible), secure the base level to the ground, and make sure everything is square and level.
On the corner that is most level, pound in a stake flush to the inside corner, leaving about 6" above the top.
Check that the boards are level, shim up any ends that need it.
Once the boards are level, drill 2 pilot holes through the board into the stake...
Then screw your board to the stake.
Go to the opposite corner (kitty corner) and check that it's all square. Pull the corner to the left or right to make it square.
Repeat the leveling and staking for all four corners.
Once it's all square and level and secured in all 4 corners, its time to reinforce. Add 1 stake in the center of each 4- ft board, drilling pilot holes, and screwing to board...
...and two more stakes on the 8 ft boards, drilling pilots, and screwing to boards.
Then, since your base is level, you can just build the 2nd level frame laying it right on top of the first, screwing through the pilot holes you drilled a few steps ago through the 8ft boards.
Then, drilling pilot holes first, screw the boards on the 2nd lvl frame to all the stakes.
Then, you fill it with good rich loamy soil, and plants your veggies. Probably not going to get too many plants in a single 8x4 foot bed, but then, you can just make more!