wood screws (suggest 3")
2 8-foot 2x4s
2 8-foot 2x6s
2 sturdy pieces of wood, about a foot in length
2 not-necessarily-sturdy pieces of wood, at least as long as the 2 short sturdy pieces
saw (table saw is helpful but not necessary)
screwdriver (or screwdriver bit for the drill) for the wood screws
Step 1: Cut the 2x4s in Half
I use a table saw for this, but any saw would work. Exact measurement isn't necessary, if you're near the 4-foot mark you'll be close enough to the middle of an 8-foot 2x4.
Step 2: Drill Pilot Holes, Then Attach the Pieces With Wood Screws
Choose a drill bit with a diameter close to that of the shaft of the wood screws you're using. The idea is to avoid splitting the wood with the screw, while still allowing the screw's threads to bite into the wood. The drill bit I used (shown) is actually a bit too small, ideally it would be a little bit bigger.
I position the 4-foot section of 2x4 on top of the 2x6 it will be attached to, and use the second 2x6 to support it so it can lie flat against the 2x6 where it will be attached. I drill 3 pilot holes (at the corners of an imagined triangle), then secure the 2x4 to the 2x6 with 3 wood screws. Then I attach one of the sturdy short pieces as shown, using three screws located in a triangular pattern: two of the screws attach the short piece to the 2x6, the other attaches it to the 2x4. Take care that the screws attaching the short piece don't try to go through the screws attaching the 8-foot 2x6 to the 4-foot 2x4. (The former 3 are perpendicular to the latter 3.)
Step 3: Be Sure the Rack Is Level
this simple design is easy to make & it's sturdy, but one drawback is that it's narrow, so you have to be sure it's level along the narrow axis (the long axis can safely be tilted; since the vertical supports are pretty strong they can take some weight).
The cross pieces at the top of the verticals (the 4-foot 2x4s) are not really necessary, they're really only there to keep the 2x4s parallel to each other. They don't take much weight or strain.