1) Begin by standing on a comfortable surface, where you have plenty of room at 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.
2) 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.
3) Then try 50-lb potato bags, extending your arms straight out from your sides and hold them there as long as you can
4) Eventually you can get to where you can lift a 100-lb potato bag in each hand – holding them with your arms straight for more than a full minute. (I'm at this level.)
... After you feel confident at level 4 - put a potato in each bag.