Well, if it's only meant to send to a songwriter, I think it has nothing to do with mastering yet. For the mix, I would recommend to mix the track just as you'd do IF there would be a vocal. So, leave enough space in the mix for the place where the vox will come. If the singer (m/f) records some scratch takes to the track, the song already sounds kinda professional, which might motivate and inspire the artist.
So, for making this premix, you need to imagine the singer's voice in the track, or just be a little aware of the energy you put in the 4 kHz regio :''). And further, follow your own intuition. There is no standard way of letting a song sound nice, so just try, try, try, and just mix the song until it invites YOU to sing.
Hmm, finally, stay away from hitting the volume fader again and again. If the artist wants to play the track louder, they have a volume knob themselves. :') Smashing things to smithereens has never brought any good, AFAIK!
