Begin by standing on a comfortable surface, where you have plenty of room at each side.

With a 1kg potato bag in each hand, extend your arms straight out from your sides and hold them there as long as you can. Try to reach a full minute, and then relax.

Each day you'll find that you can hold this position for just a bit longer. 

After a couple of weeks, move up to a 2kg potato bags. Then try 5kg potato bags and eventually try to get to where you can lift a 10kg potato bag in each hand and hold your arms straight for more than a full minute. (I'm at this level).

After you feel confident at that level, put a potato in each bag.