This has always been to my go to guard
Instructionals
Marcelo Garcia: X-Guard and his first Submission Grappling instructional (groundfighter.com)
General tips
Mantain over-under clinch
DON'T fall on your back when attempting to sweep, fall on your side, that means when you're attempting the butterfly sweep your hips should be at an angle
Mantain some distance between your hips and his, if your feet are stuck to your butt, it won't work
remember to start sitting up, using butterfly sweep while standing on one foot and one knee works really well too
Keep your body curled in a little bit as if you're doing a granby roll
Know your transitions (i.e. if he stands up and tries to move back, follow him), learn to shoot from sitting butterfly guard position, learn to switch back from standing clinch to (kneeling/standing on one foot) to butterfly sweep (and vice versa) at will