Move the jet ski into knee high or upper thigh-high water. You will need to then board the craft carefully or you will fall off. Approach the jet ski from the rear and bring one knee up to the floor of the jet ski and then bring up your other knee onto the jet ski.
If you are using a jet ski that has a safety key, insert the safety key (which is attached to a lanyard and should be attached to your wrist at all times while riding the jet ski) and turn on the engine. Press the throttle slowly to pull away from shore. If you give the jet ski too much gas too quickly, you will likely fall into the water so be easy on the throttle until you get a feel for it.
Ride on your knees for a while to get used to maneuvering the jet ski. Practice a few wide turns like this to get a feel for how the craft handles turns. Always give the jet ski more gas when making turns or the turn will usually fail.
Pull one knee up and place the sole of your foot on the floor of the jet ski. Then carefully lift yourself to a standing position, placing the other foot on the floor. For added stability, one foot should be further back, toward the rear of the craft, than the other.
Shift your weight slightly when making turns. This doesn’t mean you should lean your whole body over—do that and you will fall into the water. What it means is that jet skis respond to shifts in weight, which is one reason people fall off of them so frequently—it doesn’t take much to lose balance on a jet ski.