Level of DifficultyEasy
Time to do repairLess than 15 mins
Age of ApplianceMore than 10 years
The problem is a worn seal ring. That ring is inside the lower spray arm's spray nozzle support assembly. The wash arm spins on that assembly.
I unscrewed the lower spray nozzle popup, from the wash arm support. That part, from which the extending spray nozzle rotator pops out, simply unscrews, by hand, from the wash arm support. Then I could lift off the wash arm itself to get it out of the way for the next step.
With the wash arm removed, I could then gently pry off the flat filter cover which was underneath the wash arm (the filter cover is simply clipped on. Use a screwdriver).
Now, reaching inside where the that popup spray nozzle was screwed on, you can see and/or feel a tall skinny plastic piece in the center of it, which actually screws that wash arm support onto, well, the rest of the dishwasher.
With that "screw" removed by hand, I removed the wash arm support.
Inside that wash arm support the old ring seal can be removed and the new one inserted.
Simply reverse the process to re-assemble.
Note that it's hard to determine where, exactly, to put that ring seal, inside the wash arm support. But it seemed to me it didn't really matter as long as it was just in there.