What's the best way to propagate succulents indoors during winter?

Are you working with leaves or whole rosettes, or just trying to get whatever propagation method works?

The main thing with succulents indoors in winter is that they're basically dormant and growth slows way down, so you need to be patient and not expect quick results like you'd get in spring. Leaf propagation is probably your best bet - just twist off healthy leaves from the base, let them dry for a few days so the wound calluses over, then lay them on barely moist soil (cactus mix or regular potting soil mixed with perlite works). Keep them in a bright spot without direct sun if possible, because winter sun through a window is usually weak anyway. The tricky part is moisture - you want the soil just barely damp, not wet, because rot kills propagating leaves faster than anything else.

For water propagation or rooting in soil, keep the temperature around room temperature if you can, and don't mess with the leaves once they're laid down. Tiny roots and baby rosettes will start forming over weeks, sometimes months in winter. Once you see actual growth starting, you can increase watering slightly. If you're doing whole rosette cuttings instead, same deal - let the cut end callus for a few days, then stick it in slightly moist soil. The slower growth in winter actually works in your favor once roots form because the babies won't dry out as fast as they would in summer heat, so if you can get past that initial slow waiting period you'll have pretty decent success rates.