You'll need a hand saw or rotary saw, a table saw, a measuring tape, and a drill.
I didn't have a table saw, so I cut the boards at Techshop before doing the assembly at home.
"I made it at Techshop"
Step 1: Rip the 2x4s
Remember, 2x4s are usually more like 1.5x3.5s, so set the table saw guard to about 1.25" to get a cut down the middle.
Step 2: Lay Out the Boards
Technically you could just screw everything together here without making any more cuts. In the next step I show how I cut a few angles for neatness sake, but this is completely optional.
Step 3: Cut Some Angles and Screw Everything Together
The two angled boards will meet the two vertical boards at 67 7/8" (5 feet, 7 7/8") from the top.
Put the boards together however you like. They're long and flexible enough that even if there's a twist (like there is in my bottom horizontal board), it will turn out ok.
TIP: assemble each joint with only one screw at first (so things are free to rotate). This lets you swivel the pieces to line up correctly without having to use a square for your angles. Once everything is "pinned" in place, you can follow up with some more screws.
(calculation: each angled board is 8 feet (96") long and at a 45 degree angle. That means the short sides of the triangle are 96" * sin(45 degrees), or 67 7/8". multiply that by 2 and you get 135 3/4" for the top)
Step 4: Screw the Tops of the Frame Pieces Together, and Hang Strings
This video (not by me) shows how to attach your tomato plants to the strings.