This yoga pose going to get awkward. I also say that to set your expectations. Not because Cactus pose is a challenging pose but because it is easy to cheat and takes a bit of adjusting to find the sweet spot. Start first by following the alignment tips below and then adjust from there.
Play with it, move your arm around to feel the stretch in the front of the shoulder.
Anterior Fibers of Deltoid
Alignment and Execution Tips
Lay on your belly. Bend your right elbow with your palm flat on the ground.
Walk your right hand out to the right a few inches to open the angle of your elbow. (You can also walk your hand in if that helps you feel the stretch)
Place your left palm on the ground and roll onto your outer right hip. Relax your head towards the ground.
Make adjustments as needed but this stretch should be felt in the front of your chest and shoulder.
After about 30 seconds in this stretch, slowly release by coming back onto the belly, then switch sides.
Stretches the front of the shoulder girdle to prepare and recover from overhead movements
Counter balances the tightness in the front of the chest from slouching