for offset you find the first substantial sound (first beat of the first measure in music theory terms) and line up the first beat of the metronome (the red line) to it by increasing/decreasing offset. do not use 25% playback speed to listen for offset, its audio is not accurate in the client, use 50% or faster to get the downbeat of the song and metronome to happen at the same time.
for bpm, either find it online (not super reliable), tap t to the beat and check offset after, or guess and check (fastest once you start to recognize bpms)- if the metronome falls behind the song, increase bpm, if it gets ahead decrease.
bpm will usually be a whole number.
if the metronome gets more off as the song goes on, the bpm is wrong, if every tick of the metronome is equally off, offset is wrong. There are songs which will require more complex timing -
https://www.youtube.com/watch?v=xauZuMPgiQw&t pishifat goes more indepth (is STD video, but timing is the same across gamemodes)
most music is single bpm 4/4 time signature though and will just need one timing point
SV - involves math:
https://github.com/Eve-ning/SV-Crash-Course-LaTeX/blob/master/builds/11082018.pdf http://zardoru.github.io/sv-tools/