Here's how to use a metronome to learn to play a scale using a pick. (electric guitar style)
Using a metronome while you practice is much more than a way of playing in time. It is a tool that gives you control over all aspects of your technique and musicality and helps you focus.
You don’t have to spend hours doing it to get good fast, in fact spending too long doing it can be counter productive. 5 to 15 minutes a day is all you need as long as you challenge yourself, and concentrate until you get tired during these short focused periods.
The best way to illustrate this is to give you a couple of examples of how to use a metronome to learn something general like a riff, and learn something specific like a scale.
1. Learn the Pattern
Try and work out roughly how to play it from any guitar tab tutorial or YouTube video. Just focus on learning the order of notes and familiarising yourself with what the riff sounds like. You are not trying to perfect it at this stage, just memorise the order of notes and the fingering patterns on the neck. It doesn't matter what your picking hand is doing at this point, use a pick, use a thumb - not important yet - don't focus on this.
2. Define Technique
Once you've learned the basic pattern, now you can focus on what your right hand picking scheme is - it should probably be 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 hand is comfortable finding all the notes in the phrase or riff.
3. Design & Coordinate
Now you need to use a slow metronome to carefully play it over 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 makes you mess it up.
Ask yourself: Why did it mess up? Analyse it. You have to understand when and why your technique fails so that you can break these movements down into smaller chunks that enable you to focus ONLY on the bit that you can't do easily
Now design yourself some short exercises that repeat these small chunks over. Use a loop station to make make the exercises more fun to play.
For the final step, you'll need to play these exercises over over in slightly different ways.
4. Refine and practise
This is where the practise 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 15-20 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 in the same one position, you can move it up and down the neck, or play the exercise with different timings, first quarter notes, then quarter note triplets, then eighth notes etc.