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 arm straight out from your sides. 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 15 or 20-lb. bags. You may have to go to Costco or another "warehouse" store to find them. After you can do that, you can try 50-lb potato bags (try restaurant supply places), 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.