Tascam 424 & Scratch Vocals?

[studiodrum]

I need to record (3) tracks, (possibly 4 tracks) simultaneously, and be able to add a scratch vocals (vocals not recorded-- only heard through headphones)

So, I would like to be able to hear the vocals (and also maybe the electric guitar) in the headphones, while I’m recording the drums and bass, on TRACKS 1, 2, 3, and maybe 4.

Can anyone give me a step by step method of achieving this. . .?
Thanks guys. . .