Simply jump and alternate your feet in the air. The key is to get off the ground as quickly as possible but also jump as high as possible. Off the ground quick and in the air as high and as long as possible. True plyometrics are an involuntary reaction so the athlete should not be on the ground winding up to jump, they should just spring off the floor.
I️ usually have the athletes start with 4 sets of 20 seconds then progress them to as high as 8 sets. 8 sets was rare, 6 was more common. These are a less intensive plyo so I️ would typically put these in early in the peaking phase and then progress the kids to a more intense variation as they got closer to their big competition. For example, these would be early in the cycle and step off box jumps would be at the end.