Choose a passage or phrase that is representative of the kind of thing that you want to be able to play fast.
Set your metronome to a speed where it is easy to play.
Play the phrase 10 times. Breath deeply and slowly while you do this. If your instrument is mobile, you can take a couple of laps around the room while you do this.
Increase your metronome by 4 or 6 BPM.
Play it 10 times again.
Keep doing this until it is too fast. You know it’s too fast when you can’t do it without making mistakes. Try to push through at this speed without a mistake.
Take a rest.
Fall back by 16 or 20 bpm and work your way up again.
Take a short nap.