How to make lip sync so that the sprite only looks like its talking when its talking?

Well, what are the lips syncing with? if they aren't syncing with any kind of sound, you can just show lip positions at random. Honestly, even if there was a sound, this would still be the easiest solution.
You can make it if the person is not currently saying a word, their mouth does not animate, but when they are, the mouth does animate

I remember watching a video on automatic lip syncing, but it's very involved, and I don't think it's worth the effort for a scratch project.

I would recommend either making your own lip sync chart with every letter in the alphabet or using an existing one. Heres an example:

yes but for example when the sprite starts talking it uses the chart but when it stops talking the usage of the chart stops.

You need to use the costumes section and time it right.