Begin by standing on a comfortable surface, where you have plenty of room at each side.
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 50-lb potato bags 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.