Hello Ian,
As said above, I'm sure each LF photographer has their own way of using this principle in the field, but here is how I do it:
1. With both standards set as normal (vertically), compose your scene.
2. Focus on the far distance (bottom of gg).
3. Slowly begin tilting the front standard forward or the rear standard rearward while watching the far distance focus. Slowly focus, as needed, to keep the far distance in focus until the near distance is in focus, too. Wow, this is a lot harder to explain than to do!
4. Once you have near/far both in focus, adjust scene focus to taste.
There are a couple of caveats to the above: 1) if using back tilt, the method I described above assumes base tilts on your rear standard. If axial tilt, then simply focus sharply on the center of the gg and tilt either standard until near/far is in focus, and 2) this near/far focus assumes your subject is on a flat plane (the lake analogy above.) If you're using it to gain DoF (which you probably are!) and have vertical subject matter (like trees, for example), then where to place focus gets a bit trickier because now the area of acceptable focus is not following a standard vertical plane. It all sounds a bit complicated, but once you do it a few times you'll have it locked down.
Oh, I forgot to mention above that you'll probably have to raise/lower one of the standards (I always use the front because I don't have rear rise/fall on my camera) to re-adjust your composition.
Good luck!