With a distorted guitar, hold a power chord. Set the level for -18dbfs
With a Vocalist, have them hold a note. Set the level for -18dbfs
With instruments that have long sustained sounds, sustain it and set it for -18dbfs (or just use the VU meter on the preamp or mixer if it has one)
With drums, set the peak to -6dbfs
With instruments with a lot of attack and little sustain, set the peak level to -6dbFS.