Clamp one horizontal member at each end of the jig.
Make sure they are flush with the ends of the verticals and centered on them.
Insure they are square with the jig by using a large square.
Clamp using 1 or 2 vise grips on either side.
If you didn't need to use the drawer members for the jig, you can drill those and the wide cylinder support holes now too so you won't have to handle the material twice. See below for how to do it.
It really helps to label each member, so that if you end up unbolting the piece later, you'll know exactly where it goes and in what orientation. Mark which side will be the top of the machine, and which member goes where.