OK so if my buddy sent me one of his guitar tabs and asked me to write a bassline to it how would I go about doing it? I've been playing for awhile so I know how to play the bass, but I am kinda theoryless... I can pick out the notes though on the fretboard. So, I CAN play root... but I dont know where to put the root notes. Advice?

It's just simple Punk/Rock bass with a little more complicated guitar. IS there a guidline like
eg. root note on every 1st and 3rd beat or what?