How Should I Warm Up For Practicing Guitar?

Warm up using the very same exercises that are in your guitar practice schedule. Play them very slowly (at 20-50% of your top speed) and focus on making your technique as perfect as you can.

Your warm up time should achieve 2 goals:

1. Increase blood flow in your hands and fingers. This prepares you for faster and more demanding playing.
    
2. Focus your mind on the tasks you intend to accomplish while practicing today.

Do NOT practice mindless 1234 chromatic exercises (and other similar drills). These patterns only train your brain to go on autopilot when you practice. When your brain is disengaged, your practice time is wasted.