The outside foot frame is performed by angling your body to its side and pressing strongly with your outside foot (placed diagonally to the ground, rather than vertically) to create a kickstand to keep your body propped up. Your partner will then press gently into your top shoulder, trying to drive it into the mat, gradually increasing pressure to test your structure. You can also practice the same drill starting flat on your back and pressing your partner’s weight up off the ground. This can help develop a stronger torso to connect your hips and shoulders together with the frame of your leg.
The concept of this drill and the basic shape of it was something I learned at a Rickson Gracie seminar years ago. It was originally used to illustrate the concept of creating a whole body frame and using your outside leg posted at an angle to serve as a kickstand to support that frame by keeping you on your side, which you then use to escape side control. I figured that since the basic goal is the same in bottom half guard, I could borrow it for that purpose here.
A couple of additions I found over time to help enhance the structure are rolling the top shoulder forward and tucking the bottom shoulder under a bit. This creates a strong structure in the upper body to support weight and a better angle to keep that structure from being flattened out on the mat.