Learning to play something new on the guitar, whether it is slow or fast, involves teaching your brain to move your fingers in some new way. The goal is to teach your brain to be able to play this new piece without you having to think about it. This is a very important point. But before we start, if the passage you want to learn is long, you need to break it up into pieces that are relatively short. A passage can almost always be split up into shorter pieces. You learn each piece individually, and string them all together in the end.
So how do you train your brain to play something ‘automatically’? In order to achieve this, you start slow. You slow way down. Be sure to tap your foot or use a metronome to keep the beat. You need to make a point to keep the beat and play the piece in time, because if you don’t, your brain will get accustomed to playing it that way and you have just wasted a lot of effort.
Slow down and play it.
If you make a mistake, stop, and play the piece from the start. Again, we don’t want to teach the brain to play with a mistake in it, because it will be hard to get rid of once you have played it a couple of times.
Keep on playing the piece, at the same slow pace, until you can play it without making a mistake. Done it? Now play it again. And play it again. Play it until you can play it without mistakes at least 5 times in a row. You are teaching your brain to play this new pattern of notes. Try not to think about the individual notes. Focus instead on certain `trigger’ notes, for example, the note that lands on the first beat or third beat. Literally let your fingers play the rest.
Once you can do this it’s time to speed it up. Speed it up slowly. Play the piece at this faster pace. If you have to think about the individual notes, you are playing too fast: slow down again.
Now you repeat this process of playing at a certain pace, guiding yourself on trigger notes, letting your fingers do the work, and increasing the tempo when you can play the piece without mistakes, automatically, until you reach the desired tempo. By that time you will have trained your brain this new pattern of notes and you can play it, without error, at an amazing pace, without having to think about it.