Practicing a mass swim start in the pool is a great drill to practice if you have any race day anxiety about mass swim starts.
How to do it?
Simple: find a few friends, clear a few pool lanes and go!
Practice swimming down and back the length of the pool in close quarters with your friends. Surge ahead or slip back to escape the melee. Breathe to the opposite of a flailing arm or kicking leg and practice sighting to both look where you are going and to find less confined water space.
By practicing close up swimming in the safe environment of the pool, you will become more comfortable in mass starts in open water on race day.
Here's a link to a video I filmed demonstrating the drill at the recent She Does Tri Camp for Women in April.