Because Halloween is the most commanding holiday at Brit + Co HQ — we literally start Halloween in June — we have to properly plan, source and come up with cream-of-the-crop costume ideas, ranging from couples costumes to mom and baby to group costumes. Harley Quinn steals the show in every movie she’s in, and after a lot of research, it’s no surprise that this light-hearted, bubbly badass, who’s been known as the Joker’s GF for years, won our hearts in Suicide Squad. (Psst: There’s an entire history of the Suicide Squad that’s really fun to dive into, BTW.) Here’s how I recreated this DIY Har…