How do I learn to shred?

Hello fellow guitarists. I can play the pentatonic and the minor scale across the fretboard. I can also play some basic solos, but how do learn to shred? I love to play Metallica songs, but never really get to learn the solos as they just seem way to challenging. I often just stick to play the easy melodic parts and intro solos. So how do I start working my way up to play those fast shredding solos? Any help and tips much appreciated! :)