Begin by standing on a comfortable surface, with plenty of room on each side of you.
With a 5-lb potato bag in each hand, extend you arms straight out from you sides and hold them there as long as you can. Try to reach a full minute, and then relax.
Each day you’ll find you can hold this position for a bit longer.
After a couple of weeks, move up to 10-lb potato bags. Then try 50-lb potato bags.
Eventually, try to get to get to where you can lift a 100-lb 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.