I find writing vocals first to be the most challenging and most rewarding way;

A set of lyrics on a page suddenly becomes a tune. You play the tune using any keyboard voice at random, then set about working a drum pattern around it. Spaces open up for bridges, cadenzas and codas, and it's time to work out some basic harmonies.

This is always fun, as the voice line can suggest many harmonic variations, and you can simply throw one off-the-wall chord in there, and the implied harmonies all change.

Adding bass becomes even more fun, as you can alter all the harmonic expectations - but it starts to become a precision art at this point.

Now comes the colouring in - turning the harmonies into rhythm parts or simple lines, intertwining...

All the time, the drum track and bass need simultaneous tweaks to maintain the rhytmic, harmonic and (in the case of bass) melodic sense.

Then I add rhythm guitars and vocals. Often at this stage, a rhythm guitar part can colour a vocal line and vice versa, or a keyboard part will suddenly jump out as being more important.

Next vocal harmonies.

Sometimes I have completely stripped out harmony layers just to get the vocal harmonies I wanted. It's all sacrifice for the greater good!

Lastly, guitar or keyboard leads. These require little effort - the hard work is all in building up the overall picture that you're trying to paint. Get the cake right and it practically ices itself.

That's my "normal" approach, anyway... needless to say, I do it differently each time I write as it's more exciting that way - I never know what I'm going to come out of the studio with 

It always surprises me very much to hear about so many people that really begin writing a song or piece of music by jotting down some lyrics.  I do understand that this may be the most inspirational starting point, but I don't ever do it because it seems like I have some boundary to stay within for the music, whereas writing the riffs (on guitar), adding the keyboards, bass and drums, painting those musical textures allows me to be adventurous, then step back when the instrumental track is done, listen, and adapt lyrics to whatever it is that I have just created. This is why I always found it strange that a lead singer should help in the writing process of a group's material, but then you look at groups like Genesis, and it must have worked.

I like to start with the lyrics simply because I can then let them suggest the music - so the music expresses something in the words.

Obviously that doesn't work for instrumentals - and frequently I find that I write a nice lick or two for a song only to find that it works better in something else! I'd find it difficult to write words that express the music I write - there are too many thoughts co-existing in it simultaneously and I'd probably end up with babble. Worth a try though!

There aren't any cast-iron rules about it, of course - the best music understands but breaks the rules, IMO

Sometimes I battle with writers blocks, but once past, I actually look forward to firing up my Cubase program and creating some sonic mayhem. Improvisation plays a GIGANTIC part in my writing and musical concept. Though having learned how to do the one-person-band thing, I much prefer playing with other musicians. Recently, I did hook up witha great bassist and we've started to develop a playing/creating chemistry.

One example for me was the song "Ghost of A Train". It started with having a drummer come over to my house one night and recording about a half-hour of improvised drumming with some suggestions here and there (though not much). Several months later, I dug out the CD of him and loaded it into the computer, before long, one of his improvs triggered a full blown idea.

Other times., I did the programmed drums/ mapping out nearly every note thing too ("Upstairs" and "Commerciality At Last" as 2 examples) Luckily, I had a friend program those who is areal legit drummer so that helped on the realism end.

Since I'm just a bassist, and I also own a keyboard (126 MIDI samples) and my dad owns a plethora of guitars (acoustic, electric, acoustic-electric), I'll often jot down stuff I come up with on music lined paper. Unfortunately, all our instruments are in one room, and our computer is in another, so it's a real pain to record...I don't have a band yet either, so that makes it harder, and I'd have to layer everything.

But that aside, I've written down pages of music ideas, most of them bass lines and "starting points" for songs, but quite a few are acoustic guitar parts. They're practically all in odd times too, and some go with each other...I just have yet to record them. But I don't have a drumset, and couldn't play 'em if I did...and I don't know how to program drums (I'm not even sure it'll do odd time signatures). As for lyrics, I find poetry often restricts the music. It's better to write music and lyrics together, so I have a notebook where I'll scrawl down any ideas, quotes, or verses for later use when I get a band. So really I'm just accumulating musical ideas for later, when I get a band.