Test with a spoon: Dip a spoon in—if the soap runs off in a slow ribbon, you’re ready to swirl. Keep an eye on the clock: Don’t wait too long, or you’ll get clumps instead of swirls.

Faiola, who made her first soap more than 20 years ago when she was 16, has refined her craft over the decades, sharing her skills in how-to sessions via YouTube, blogs, classes, social media and ...