Begin by standing on a comfortable surface, where you have plenty of room at each side.
With a 5- kg 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 10- kg potato bags.
Then try 50- kg potato bags and then eventually try to get to where you can lift a 100- kg 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.