No mobility work for me. Though I probably should do some.
I was doing Limber 11 pre workout some time ago, but stopped because it was just wearing me out before my workout. I usually just do a general warm up on an stationary bike, then jump right into movement specific warmups.
For squat, specifically, I take a little more warm up with lighter weight to find the groove and get to depth. Then pile on the weight. My right ankle isn't as mobile, dorsiflexion wise, as my other one but my Olympic shoes with 3/4 heel help that quite a bit.
Speaking of shoes, if you squat a lot I'd highly recommend a proper lifting shoe of some sort. Heel height it up to you, but something with a non compressible sole and straps. Its light years better than squatting in trainers.