Begin by standing on a comfortable surface where you have plenty of room at each side.
With a 5lb 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 that you can hold this position for just a bit longer.
After a couple of weeks, move up to 10lb potato bags.
Then move up to 50lb potato bags, and then eventually try to get to where you can lift a 100lb potato bag in each hand and hold your arms straight for more than a full minute. (I’m at this level now.)
After you feel confident at that level, put one potato in each sack.