Begin by standing on a comfortable surface where you have plenty of room on 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 you can hold this position for just a bit longer. After a couple of weeks, move up to 10-lb. potato bag. Then try 50-lb. potato bag and eventually 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 this level, put a potato in each bag.