Begin by standing on a comfortable surface, where you have plenty of room at each side . . . pretend your on stage at the Apollo.
With a 5-pound potato bag in each hand, extend your arms straight out from your sides and hold that position as long as you can. Try to reach one full minute, then relax your arms to your side.
Each day you'll find that you can hold this position for just a bit longer. After a couple weeks, move up to 10-pound potato bags.
Then try 50-pound potato bags, and eventually try to get to where you can lift a 100-pound potato bag in each hand, and hold your arms straight out to the side for more than a full minute.
After you feel comfortable at this level, put a potato in each bag.