Stick a cutting in damp soil.

It is easy.

Take a 4–6 inch stem cutting from a healthy citronella plant, right below a leaf node. Strip off the lower leaves so only two or three remain at the top. Stick the cut end into moist potting mix—no rooting hormone needed, but it helps if you have it.

Keep the soil consistently damp and put the pot in a warm spot with bright indirect light. Covering it with a plastic bag or a clear cup creates a mini greenhouse, which speeds things up. Roots should show in 2–3 weeks.

Honestly, you will have better luck with this than keeping the mother plant alive through winter indoors.