Repetition.
Practice it 100-200 times.
No, I'm not kidding. If you are someone who works out everyday, you know that repetition is the key to gaining strength. The same can be said about practicing your guitar. Lately, I have been seeing many of my students genuinely putting in the time to practice their guitar lessons. They have been working on their songs and their exercises; and they are beginning to see the fruition of their efforts. As their instructor, this is a great reward for me.
My job is to show my students how it can be done and how much progress can be made in a short period of time - provided they follow my instructions. I tell them to think in terms of repetition and practicing a song or a scale or a stretching exercise, as a daily ritual; and they have been doing exactly that! If you are working on a series of songs, there are steps that need to be taken to truly learn how to play each song.
1) You need to know and learn how to play all of the chords in the song.
This means practicing the song by yourself by reading and understanding the chord charts, changing from chord to chord, and making each chord sound clean with no buzzing from the strings, and no shortcuts and no cheats. Do combinations of two chords that you struggle to play. For example: Practice playing the C and F chords using a metronome set at around 60 beats per minute. That setting is a sufficiently slow enough tempo to be able to change both chords clearly in rhythm. Try not to go too much slower than that because ultimately, you will need to be able to change the chords clearly at faster tempos.
2) Speaking of using a metronome.
If you are not actually practicing along with the song, you must use a metronome. Not using a metronome is quite simply, counter-productive. Building your mental meter, your inner clock, can take a few months or a few years. Consistent tempo in practicing and playing your instrument is paramount. It isn't just about strumming through the chords of a song, just to prove that it can be done. It is about creating music with those chords and making it your song, your interpretation.
3) Speaking of playing along with the song.
Once you have learned all of the chords and you are able to change the chords clearly in rhythm, set aside the metronome and start playing along with the song you are trying to learn. There are structures to songs that you need to understand and memorize. All of the little changes, all of the little nuances, all of the little signatures that make that song interesting to you and your listener. Learn the beginnings and the ends of songs.
When you practice along with the song you are learning, the music you are listening to must be louder than you. (I suggest using headphones or earplugs for this). If you are able to play the music loudly enough so as not to disturb others, this would be the ideal way to practice. Keep in mind, you are following the music - the music is not following you. This means, if you lose your place in the song, catch up or be ready for the next measure. Never stop playing just because you missed a part.
Also, remember to use “Dynamics” Not everything you play needs to be on “10”. Think in terms of every chord or note you play, having its own volume. Subtleties in creativity, interpretation and nuance, can make or break a song.
If you do these things as a regular practicing method, you will begin to use these techniques as a normal habit for learning many things on your guitar. It is all about repetition and genuinely putting in the time to practice.
Remember: Playing is what you know. Practicing is what you don't know. Don't practice until you get it right. Practice until you can't get it wrong.