Beginning with a basic combat base entry into deep half guard, start by driving into your partner with your head until they push back into you. Once they push back, dive underneath them (again, consider checking out the deep half entries from before). You have all of your basic sweep options from here, depending on your partner's posture, but if you specifically want the back, there are some steps you can take to help ensure that happens.

Start by untucking their lapel with your right arm (the one that's furthest under your partner, and nearest to your head). Grip the lapel with your thumb down. This grip will come in handy (pun intended) momentarily. Now turn your hips to your left, causing your partner's weight to shift forward. Once they post on the mat forward, you can sneak out the back door by inserting your right hook behind your partner's knee.

Note that the lapel grip keeps your partner from turning away (and, ultimately, back toward you).