Measure twice and cut once. Lay all your measurements out before cutting anything to get the maximum out of each board. If you have access to a power miter saw it will make your life a lot easier, if not a table saw works well as does using a framing square as a guide or fence for a skill saw to maintain straight cuts.
+1 on measure twice cut once. It also helps to put it on paper before you start building. Sketch or drawing. Here's a pic of the frame work on my recent build. Notice how all the outer top members are supported by the strong back legs. I also layed a piece of acrylic down on the base and painted the under side of it. This will help with clean up. Easy to wipe up spills and such.
Spend the money and purchase a pocket hole kit from lowes. Instead of toe nailing or trying to find the right angle it's all done for you; plus pocket hole joints are far stronger than the conventional. They also grab and squeeze the joint together so make sure your cuts are straight. Once you use a pocket hole system you'll never build anything without it.
Like Speedo2wet said, get these. You can even make fake shaker panel doors with it if you have access to a table saw as well. The clamp snaps in the end of the jig and you just, clamp, drill and I even clamp the two pieces of wood together with it to make sure they are flush when screwing. These things work great. Best 50 bucks you'll spend for wood working.
What size tank do you have. Honestly with one of these jigs you can build it out of 1X4 and 1X6. It will give you more room under the tank and you don't have to skin them.
If it's a four foot tank or less, everything can be 2 x 4's. If the tank is greater than 4 feet, the top frame needs to be 2 x 6's. No center brace needed. And just the heads up, this design is overkill and can support a Sherman tank.