I'll get right down to it. I've only had to write vocals for a song maybe.. Once before. It did not go well. A friend of mine has written his own little instrumental and would like me to write and perform vocals for it.
I would 'love' to do this for him, but have no idea how to go about it. He understands this and would like me to give it my best shot anyway. If it goes well, we'd hopefully work together more in the future.

I've went and listened to instrumentals of songs and gone, "Okay, this is where I feel like the vocals should be and how they should sound." I find that I (And please excuse me if I use the incorrect terminology here. I don't know the terms) sort of try to sing (Or growl as the case may be) or just place each syllable of each vocal onto each individual.. I want to say note? It's not every single note, but essentially I try to follow the rises and falls of the instrumental with my voice exactly. I'm not sure if this is physically possible to do, first of all. And second of all, it isn't what you hear in songs today. Or ever.

So the question is essentially.. How do you write vocals to actually fit over a pre-written track? (I word it like that specifically because most of the advice usually amounts to, "The vocals are generally written before the instrumental". I'm not sure if that is actually the case, but that's not what's going on here.)
So can anyone give me a hand?
There is no logic to it. All you can do really, is listen and think of a melody to complement. Following the feel of music will generally sound generic and boring. What you want, is something to complement the music, not necessarily follow it. But sometimes people do follow it.

Max Martin is one of the most successful songwriters of all time, and he did that in pink's "so what". So, you can definitely do it, if that's the sound you want, but it sounds like that.
I guess i can explain how i do it, since there is no actuall set way to go about it.

I start with a pretty much finished song, instrumental wise. I know what parts are the verse, chorus, bridge and such, but not much more, maybe just a bit of melody or whatever. So once i know where the different parts are, i basicly just start singing over it, trying out different melodies. I dont write the actual lyrics yet, just go with some random nonsense words that dont make a lot of sense. Once i have the melody down, or most of it, i start writing lyrics that fit to it. And then there is a lot of tweaking, like maybe puting the vocals on places that i didnt intend at the start, and changing the words that dont fit and such, until im kinda satisfied with it. And thats all there is to it. I've done it this way for all my songs, and for songs that other people wrote, and i just sang on them, and it seems to work pretty ok.
Joža je kul. On ma sirove z dodatki pa hambije.
What gorkyporky posted is pretty common technique for lyricists. Spit out some nonsense words to get a melody or flow going, listening to a section over and over helps, then fill in the syllables with real words as you write actual lyrics.
Gorkyporky offered solid advice on developing the vocal parts of a song. I do it much along the same lines. Listen to sections over and over, forget about what you think you should write and focus on the melody. Keep trying new lines, without regard for the lyrical content at this point. If your lyrics are "oh the patio on the grass black burn and the summer gone potato mash", it doesn't matter. Find the melody. Then start adding the words.

Once I have a basic idea of the melody I want, I'll then take a break to figure out what exactly I'm writing about. From there, it's really up to you and your feelings to come up with the lyrical content. If the song speaks to you in a certain way about a certain thing, focus on that and see what develops. It's not all too different from poetry.

The melody comes first, though. Don't forget that you can always revise it later. It's a process, not something you sit down and just do. Practice, and take your time.
Hey everyone. Thanks so much for all the advice! He didn't end up sending the track, and is instead going to be bringing it in person.

I decided I might as well practice the advice provided, though. Grabbed a track off of youtube.
I found it was very easy to 'get into' it, and just blabber while not thinking about what I was saying. I can see the merit in the method, and will likely be using. (And, aside, it's alot of fun to just 'let go' and see what feels like it fits together.)

So yeah, again, thank you! Really appreciate it.