섞다 is mixing separate things into one, and -게 supplies the manner — here almost always 이렇게 (like this, said while demonstrating) or 어떻게 (how do I?). It works for food and drink (이 소스를 잘 이렇게 섞어서) and equally for blending two things that are not physical: 발레랑 필라테스를 어떻게 이렇게 섞은 운동 (some sort of exercise mixing ballet and Pilates). Contrast 섞이다, where things mix without anyone stirring.