In the example video above, I am demonstrating this method using a simple pentatonic scale fragment from a longer lead guitar riff.
My goal is to play the riff at 144 bpm with 16th notes - that's quite fast, so I will set the metronome at that tempo and work up to that speed by playing 1/4 notes, 1/4 note triplets, 1/8th notes, 1/8th note triplets and finally 16th notes.
Let's say you start by playing the riff using 1/4 note alternate picking - slow and steady. That soon becomes mindless and easy, so this is when you should focus your attention on dynamics and rhythmic changes.
This method is about learning to play something new - a riff or a phrase, even new chord. This method helps you absorb all the techniques within the phrase you are learning so that you advance your core guitar skills permanently.
By learning something methodically you'll be able to master different playing techniques quickly by avoiding muscle memory during practice.
First, try and figure out roughly how to play it from any tutorial in a book, magazine or YouTube video - There are lots out there and at this stage it doesn't matter too much which one you use just to pick up the shape or the pattern.
Familiarise yourself with where the notes are and what the riff sounds like.
Next, decide what your right hand picking scheme is - it's a good idea to use alternate picking (up and down-strokes) , or if you prefer, just down-strokes. Adjust your left hand so that your fingers are close to the strings and your hand is comfortable finding all the notes in the phrase or riff. You shouldn't feel any stiffness or tension in your hands, arms, neck or shoulders, and make sure you don't forget to breathe.
Now you need to use a slow metronome to carefully play it through a few times. The goal of this step is to coordinate the right hand pick strokes with the left hand fretted notes.
Once you've played it repeatedly 5 or 10 times SLOWLY, increase the metronome tempo slightly and repeat.
Keep doing this until you get to a tempo that is too fast, forcing you to mess it up.
Ask yourself: Why did it mess up?
You have to understand when and why your technique fails so that you can break these movements down into smaller chunks that help you to focus ONLY on the movements that you can't do easily
Now you have to design yourself some short exercises that repeat these small chunks over.
This is where the practice starts!
Now you've isolated your technique weakness and designed yourself some exercises that focus on fixing these problems and getting better.
Set the metronome at 80 bpm and play each note of the riff on each click of the metronome. You must be able to play the exercise perfectly at a speed that is comfortable for you 5 -10 times in a row without a mistake.
If 80 bpm is too fast, just slow the metronome down a little.
Once it becomes easy, increase to 100 bpm until you can play it perfectly 5 -10 times in a row without a single mistake.
Then continue to increase metronome by 10-20 bpm and repeat the process. So this until it becomes too difficult, then stop.
Everyone has a speed limit so don't worry about stopping when it gets too hard - you can develop speed later with different exercises that tackle the mechanics of playing fast.
For quickest results, rather than play the exercise the same way, apply different techniques or play the exercise with different timings, first quarter notes, then quarter note triplets, then eighth notes etc.