Unfortunately, most of us have lives! As much as we'd like to immerse ourselves in the musical process, our obligations, annoyances and just plain fatigue take their share of our time. So maximizing the time we do steal from something else is really important. Here is a basic outline of a practice routine. It may not work for everyone and you very well may want to adapt it to your own musical path. But at least it's a starting point!
Say you have an hour to practice each day. This may seem like a lot of time to some and not nearly enough to others so you can adapt this accordingly. I'm also assuming I'm talking to a student or recreational player who has some experience. A raw beginner will need another course of action entirely.
The first thing to do when you pick up the guitar is tune it! Seems basic and simple but I've seen many a player dive right in and I can tell you that no matter how well you play a piece of music, and out-of-tune guitar will make it sound bad. After you're in tune begin playing an exercise or two, something like a very basic 1st position diatonic or chromatic scale. Start slowly but always keep a steady beat, gradually increasing the tempo as you ascend and descend repeatedly and without stopping. I'm assuming you're using a flat pick to do these scales and be sure to alternate the direction of the pick from one not to the next.
Practicing scales has both advantages and disadvantages. They definitely will warm you up and in the longer term, help with strength and learning to use fingers independently of each other. The down side is that they can end up taking way too much of your precious practice time as you learn more and more of them. Also, advanced players with are into improvising will tell you that implanting the sound of a diatonic scale on your brain can in some ways stifle the creative/improvisational process. So give those scales just a few minutes, certainly no more than 10% of the time you have to practice.
Next, try playing through a song or two (or more!) that you know you can play fairly well. This builds confidence, plus it's fun! No matter how "simple" or non-technical a song may be, playing it and playing it well pays off both in the short term and long by reinforcing important musical concepts. Give this portion of your practice routine at least 50% of the time you'll be playing.
Then it's time to work on more challenging stuff. If you're taking lessons, that should be your latest lesson or a recent piece that was giving you trouble - maybe even a couple of those types of songs. Your teacher understands that you cannot be expected to play a piece perfectly in a week or two so even if you've done fairly well on your most recent lesson there surely will be small elements that need polishing up. Don't get frustrated with your latest piece and don't give up if it's giving you trouble. The trick is to break it down into smaller sections, keeping in mind that the most basic concept you're trying to conquer is rhythmic consistency, i.e., moving from one measure to the next or even within a measure without stopping. This may mean you only get through a line or two of a song but ultimately that will pay dividends. It is far, far better to do this than to pracitce a piece from beginning to end over and over, stopping at the same places each time, with the idea that if you play that piece a hundred times those stops will go away. They will not, and you'll just end up practicing your mistakes! Give this most challenging part of your practice session perhaps a third of the time you're playing.
Then, go back to the fun stuff! Play those easier tunes again or perhaps some different ones that you've also played well in the past. Remember, the whole reason you started playing in the first place was that it looked like fun - and it should be.
Don't be surprised if you look up at the clock and find you've played quite a bit longer than you intended. But if you find that the real world intrudes and you can't play as long as you'd like, make a vow to play longer next time! Again, remember that what you're doing is exercise, plain and simple. And like all exercise, a moderate amount on a frequent basis is far more beneficial than a LOT on an infrequent basis.
Good luck and enjoy!!!
Peace & good music,