Begin by standing on a comfortable surface where you have plenty of room at each side.
Then with a 2.5kg 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 5kg potato bags.
Then try 10kg potato bags, and then eventually try to get where you can lift a 20kg potato bag in each hand, and hold your arms straight out for more than a full minute.
After you feel confident at that level ............ put a potato in each bag!