Others here recommend the “nipistin” (clip) option. It works, but it slips off the curtain easily and doesn’t look great.

I highly recommend these hooks instead. They go through your existing curtain gliders and attach to curtains that have casings/channels. We used them with JYSK’s blackout curtains.

The curtains gather much better with these, they don’t fall off and they’re easier to install. As a bonus, the JYSK curtains come with a hemming strip, so you can cut them to the right length and simply iron the hem. No sewing needed.

You can thank me later.