Get your setup ready. Plug the guitar into the pedal(s) and the pedal(s) into the amp.
Find the desired pickup setting. Bridge pickups usually tend to give the heaviest and most aggressive sound but this could vary from person to person and guitar to guitar.
Metallica's sound, like most metal, comes from "scooped mids." A scooped mid refers to when the treble and bass knobs are raised while the mids are significantly lowered.
This is the general sound Kirk Hammett goes for.
If your pedal has a noise gate, try it! If there is too much noise with your gain turned up, it might be a good idea to use it. Noise gates generally cut down sustain, so the less you use it, the more natural your sound will be.
However, compromises do need to be made on certain occasions.
Learn the desired songs. 'Master of Puppets' and 'The Four Horseman' are great for metal pedals, though extremely difficult to play.
Songs like "The Memory Remains", or "Fuel", or even "Nothing Else Matters" are great for beginners (though beginners for Metallica is somewhat advanced).
Depending on the song, choose the right pedal. Anything off of the albums 'Kill 'em All', 'Master of Puppets', or 'And Justice For All' really needs a metal pedal to sound great.
Anything from 'Black Album' until now can work with a distortion or overdrive pedal.
Metallica plays fast. This means you need to use a pick heavier than .70 mm or else your arms will tire and struggle to keep up.
If unsure, try different gauge picks out. Some only say heavy or extra heavy. Pick what works best for you, though.
Kirk Hammett uses a .88 mm pick, which has a great overall thickness and works with the fast, yet intricate, nature of Metallica's sound.