Don’t complicate the breathing, and don’t try to push your belly out against the belt. Simply take a breath, hold it, tighten your abs as if you’re about to take a punch or as if you’re constipated, and then continue holding that breath throughout the entire rep. You can do this with either a closed mouth or an open mouth - you shouldn’t be holding your breath against your lips with puffed out cheeks. Instead, you hold your breath against a closed glottis, which is located in your throat. Say the word “hick” out loud and hold the “ck” sound at the end - that’s how you do it.