The Single KB Front Squat has a ton of variations. When you hold it in a manner so the bottom of the KB is up, it's called a Bottoms Up Squat. Regardless of the variation, you drive through your heels and keep the KB in close to your chest (without knocking out your teeth). Imagine hugging the KB and doing an air squat.
For Double KB Front Squats: Put two KBs in the rack position, and do the squats.