It is easy to accidentally get paint on fabric curtains when you are painting anywhere near them. Removing the paint stain is possible, and you can do so with materials you probably have on hand. Try to remove the stain as quickly as you can. The longer it sets, the more work you will have to do to get it out of the fabric.
Scrape off any excess paint with a butter knife or other sharp edge.
Lift the curtain rod off its supports and pull the curtain off the rod. Take it into the kitchen or bathroom, but don't let the stained part drag on the floor or touch the rest of the clean fabric. Rinse the stain under warm water.
Fill the bucket with one part warm water and one part liquid dish soap. Agitate the water with the sponge.
Press the sponge into the paint stain gently. Dip the sponge back in the water and sponge the stain again. Rinse the curtain in warm water. Repeat the sponging and rinsing until the stain is gone, or no more paint washes out of the fabric. Rinse out the sponge.
Tip a bottle of acetone onto the sponge to wet it. Blot any remaining stain with the acetone until it comes out. Rinse the curtain under warm water.