On warm days in the spring/summer warming it by hand is fine.
On really cold days I have microwaved it first as it was the only way to get it soft enough, most of the time cup of water is fine.
Splitting the bar in 3 is must, if you drop a bit chuck it. When you have used it on the piiant, then use that bit on the glass and finally your wheels. They caly must be soft, keeping kneading it to reval a clean face/side at all times.
I prefer to use the clay with soap and water ratrher than QD, wah and rince the car but don't dry. Pre Wet the panel with a wash mitt and soap, use some water in a spray bottle with a few caps of car shampoo, spray that on the panel and clay away, using very light pressure, enough just to keep the clay on the paint surface.