Begin by standing on a comfortable surface, where you have plenty of room at each side.
With a 5-lb 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 10-lb potato bags.
Then try 50-lb potato bags and then eventually try to get to where you can lift a 100-lb 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.