first cut 2x4s in half
take 2 2x4s measure and make a mark half way down both of them . then center them and bolt ( you can use nails but bolts work better) them together.
repeat for other side.
you should have two giant X's
it doesn't matter which way they go,so pick one side as the bottom measure up 3 inches from each foot for the bottom braces.
now attach your other 2 boards with nails or screw them together.
your frame is now done and you can start to work on the barrel.
you can cut your bottom braces a few inches longer than the barrel for a better fit but you don't have to .