it exactly at the ends of the legs, it made more sense to me to measure up 1 inch from the bottom of the legs. it seems like it will make it stronger, plus it keeps all that extra wood surface area off of the floor. if there is a bump in the floor it would be wobbly.
smear glue on the end pieces, flip them over, stick them down, drill your pilot holes, screw in your screws and square it all up, just as you did for the other side.
repeat this process to make an identical flat frame as is seen in the second pic.
set your 2 flat frames on 