There is a significant difference between a sandbag and the conventional weighs though; the sandbag changes its shape during the workout, which means the effort you are putting is dynamic (you'll be changing the force you use during the lift), more stabilizing muscles are used to
control the bag (including core muscles) and you are working on your grip at the same time because the bag is harder to hold
than a dumbbell or
barbell.
Use standard
barbell press technique; don't splay elbows out to the side excessively, use shoulder width grip, use a tight grip, keep wrists as straight as possible, bring the weight down to the chest using full range of motion or what your flexibility allows without pain, lower the weight down slow and press up faster, contract abs, glutes, and chest simultaneously while pressing, slower the weight down
controlled and slower
than you are pressing up, inhale on the way down and exhale while pressing up.