#1
i want to write a huge piece of instrumental music with loads of different layers and sounds.

i want to use a few different instruments (well basically guitars, bass, keyboard and drums).

i want it to cover many different styles of music and guitar playing.

but i don't know where to start, can anyone give me some pointers please.

#2
You got me laughing...

You want a huge instrumental piece with "loads of different layers and sounds"

Yet!

You only intend to use the most basic instruments (you can get great pieces by black metal bands using just those).

Its not going to be that huge.

Anyways, where to start?

Play jazz guitar with heavy distortion.
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#3
Start with one of the following:

A harmony
A melody
A rhythm

For example, once you get a melody, start a secondary melody if you will, polyphony. Theres infinite approaches, really.

Since it's gonna be a long piece, you have to set out a structure. Study classical structure, how movements should follow each other, etc.
#5
Start by studying theory, learning and analysing songs in the style you want to write in and practicing composition.
#6
Listen to teh classicals.

I find a good symphony to be very inspirational. It's awesome hearing out each instrument adds the each movement to convey different feelings. I'm actually (kind've) writing a symphony. I was going to go for the crazy idea of getting it done by the end of the summer but i think a year is a reasonable amount of time.

Goodluck TS. Writing music is very rewarding.
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#8
You may want to look into counterpoint and voice leading. Even with just guitars bass and keyboard you can have a lot of voices.
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#9
If you want it to cover many styles, listen to and analyze many styles. There'll be things you pick up in one style that could give a little flavor to another. In the analysis, you're also bound to come across a couple different tonalities that might make you think "Oh, I never thought of that!" and you'll be able to integrate them.