The 19mm bolt on the tensioner is the only 19mm bolt in that area so there's no fear of touching the wrong bolt. It's roughly between the idler pulley and power steering pulley, but it's not on a pulley itself.
Loosen this bolt two to three turns. Just so you can turn it by hand, and no further. You don't want it to come off!
The tensioning nut is on top of the tensioner. It points down into the tensioner behind the power steering pump. It's a hollow tube a couple of inches long with the top part shaped like a hexagon to accept a 13mm wrench.
The only indication that you've slackened the tensioner is that the tension indicator will no longer be at the proper position... assuming it was at the proper position to begin with, and the belt will be looser. It'll still be on there pretty good, but you'll notice it's not as tight.
You still have to wrestle with the tensioner pulley, the small one to the US passenger side of the 19mm bolt, which will now be relatively loose, to get the old belt off and the new belt on.
If you haven't replaced a belt on an M103 before, follow Stan's advice and remove the fan and clutch. If you don't know how to do that, let us know because it'll probably fill another page.