How to use the "instant alpha" in Preview

I overlaid a rectangle atop an existing PNG image in Preview:

After enabling the Markup toolbar we can select Instant Alpha

Now what? I tried to get a selection tool to mark the rectangle but no selecting action occurs. The instructions I was looking at are here https://discussions.apple.com/thread/6774112

Solution 1:

The way Instant Alpha works is that it selects color that are similar to it. It's not as nice as Photoshop where you can select multiple areas and get really granular with the selection criteria so you'll have to do multiple steps.

1. Select the Instant Alpha tool
2. Using your mouse, select an area you'd like to remove
3. Slowly drag your mouse down and select an area you'd like to remove. If you select too much, back off until you've selected as much as you can
4. Hit the ⌫ Delete key
5. Repeat until you're removed the background completely.

(I may have gone too far on the first background selection, but you'll get the idea)