It is without doubt the biggest area that guitar students struggle with... making time to practice, and when they pick up their guitar, what to practice first.
It may seem obvious but the best way is to make a list. Keep it simple, just 5 things, in order of priority, and don't do other things on the list until the first priority item is done. When you've finished practicing immediately write out your list for your next lesson.
A bit like this article in fact, from original productivity consultant Ivy Ledbetter Lee in 1900:
100 year old to-do list hack that works like a charm