How do I use diaper liners?

Hey all! I’m relatively new to cloth diapering- I’m a FTM with an almost 8 month old. When he was born we bought a couple of boxes of cloth diapering supplies from a woman who was done with them. We got a bunch of covers, inserts, a toilet sprayer, and some disposable liners. I’ve never used the liners before. When he was a newborn I just tossed the diapers directly in the wash, then at 6 months when we introduced solids I started using the toilet sprayer. Now that his poops are getting more solid, I’ve been thinking about using the liners.

Does anyone have experience with the liners? Reusable or disposable? Do you find them useful? Do you use one every time? What if he only pees, then I wasted one?

Thank you!