When dismounting on the left, swing your right leg over the saddle and then between your left leg and the bike. You'll be able to start running straight away. If you don't do this you'll have to do a skip before you can run. Pick the bike up by the top tube and slide it onto your shoulder. Run across the obstacle and put your bike down, maintaining momentum.
When remounting, land on the saddle with your inner thigh and then slide on to the saddle.
You should be aiming to take 6-8 steps before getting back on, the less the better.
Try practicing this until you get it right.
Don't worry if you fall off or even crash, you will get better. Practice on grass obviously, it hurts less when you fall.