Break your practice time up into thirds. Even if you only have 15 minutes to spend on your instrument, 5 minutes of practicing each section correctly will yield great results.
Just like an athlete, you are using muscles over and over so it's a great idea to warm them up. I know, younger players will say they don't need to. Let me tell you, after playing guitar for 20 plus years, warming up will make all the difference in the world. Best way I've found: chords. For beginners, just take two chords that you know and switch between them for say 20-30 repetitions. Switch chords and repeat. More advanced players use Barre Chords, or drop some music theory in the mix and switch chords using the circle of 5ths.
There's no denying that at some point you will be using scales in your playing. Again, repetition is the key to learning. Pick any scale and practice it in every key. Whether it's the tried and true Minor Pentatonic or a more advanced Harmonic Minor, play them in every key! Play them low on the neck where the frets are wider. Play them higher where the frets are smaller. Mixing them up gets your fingers and muscles more comfortable all over the neck. Pro tip: If you want to play fast, you have to practice slow.
Yes, the part of practice that everyone loves. During the first two parts of practice your focus was on Great Sounding Chords and Scales. You were paying attention to finger placement on the neck. Your chords were sounding out clearly with no muted notes and no buzzing. Your scales were smooth and fluent. Now that you put all that effort into working on your technique, let's hear what you can do. Start jamming to your favorite songs. There are plenty of backing tracks out there on YouTube and other sites that will allow you to play along with a full band with the guitars removed.
Consistency is the key. Play every day! Remember that even 10 minutes a day is better than putting in an hour once a week. If you can, get a guitar stand and keep the guitar out of it's case. You're more inclined to pick it up and play if you can see it. (This works great for the office too.) Nothing can the relieve stress of a job like closing the door for 10 minutes and playing guitar.
Matt Brechbiel has been teaching students of all ages and levels to play the guitar for over 20 years. Book a online guitar lesson with him or visit him at mattbmusic.com.