Most sandbag strength movements require bio-mechanical breathing where inhaling is performed during the eccentric phase of a movement, and exhaling is performed during the concentric phase.
For example, with a shoulder press you would inhale during the lowering phase and exhale during the lifting phase.
Breath as calmly and naturally as possible with a rhythmic inhale through the nose and an exhale through the mouth.
When breathing focus on your exhales and let the inhales take care of themselves. If you focus on inhaling, it’s easy to hold your breath which we don’t want to do.
Some movements require an extra breathe, for example clean and press.
During the clean and press there are two concentric phases, the clean and then the press. Take a 2nd breath between the two concentric phases whilst the bag is in the rack position.