Add the stabilizer right away, because until you get dissolved stabilizer showing up in the water, you're going to have to add bleach daily to maintain at least 1 ppm.

It takes a long time for the stabilizer to fully dissolve. Matt said to retest for it after 2 days. I'm saying you need to wait longer than that, because in my experience it takes more like 5 days or sometimes longer to fully dissolve. If you re-test too early, you'll add more, and then when it all finishes dissolving you're likely to find that you've added too much. Since the only way to lower it is to drain water and refill, you're much better off to sneak up on the target number (I'd say 20-40 ppm) than to try to hit it all at once and overshoot. It's your call, it's your pool...my advice is to be a little conservative.

And you can either put the 3 quarts in the skimmer, or pour it slowly in front of a return--either way works fine. Just be careful not to splash it on you or the pool edge.

Janet