I'm sure there's been tons of threads on this but does anyone have any advice for writing a singing part over riffs that you already have?
For how can I give the King his place of worth above all else
when I spend my time striving to place the crown upon myself?
try to just sing anything that comes out, it might be nonsense and skat-ing, but it gives you a foundation to build lyrics onto. If you don't like how it turned out, just restart the song and try it again until you do.
This should be in techniques I believe.

Anyway, I usually, like the above poster, sing the first thing that pops out, and roll with it, then when I hit a wall I refer to my, journals and notebooks, which I write in daily. Then I start dissecting pieces to fit together, until I have a 'Frankenstein' of a song that makes no sense to anyone. Done!

So I suggest writing constantly in a notebook, thoughts, un-thoughts, stream of conscious, daily. That way you will have plenty of ammunition when the time/riff comes.
"Pain or damage don't end the world nor despair, nor fuckin' beatings. The world ends when you're dead, until then you have more punishment in store. Stand it like a man, and give some back."
I generally strip what I'm playing down to it's base chords and then sing over it, otherwise the melody I'm singing gets too affected by what I'm playing.
Quote by Kozlic
Music doesnt need to be quality to be good.

Press eject, give me the tape, if you know what's good for you
okay...i've had quite some trouble with the same thing, i probably have about an hours worth of original music, but only 3 of those songs have lyrics....and i have found a working method

Step One:
Go Buy Some Beer

Step Two:
If you haven't already, record this riff onto something, an iphone, whatever...anything will work at this point. Listen to your riff for the duration of a beer....

Step Three:
Identify what a Song Sounds Like... Like imagine Drowning Pool's "Bodies" without lyrics...obviously doesn't sound like a love son, Sounds Angry...That gives you a corner stone to build off of.

Step Four:
Turn The Music OFF... Open another beer.
Open up a word document and just write out angry stuff...doesn't matter if it goes with the song or not...just write, as much as you can, whether is moves around from hating your neighbor to hybrid cars...just keep writing.

Step 5
Turn the Music Back on, Skim over your page of everything you write, and like Distortyourself said, Just start singing to it, do this a few times, certain phrases will pop up at you, or atleast one. One works just fine, cause that gives you a mood, words, rhymes, feel to build off of....and try to take it from there. If that doesnt work. just finish your beer. you still win.