Begin by standing on a comfortable surface with 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 maintaining this for a full minute, then relax.
Each day you’ll find that you can hold this position a bit longer. After two weeks, try using 10 lb potato bags. In two or three more weeks you’ll probably be ready for 50 lb potato bags. You’ll be surprised at how soon you will be able to lift a 100 lb potato bag in each hand and hold your arms straight for more than a minute. (I’m at this level.)
When you feel confident enough, put a potato in each bag.