well you basically just need the amp, if you have 12V available then you can go for the bigger 2x10W one.
Then you need a Jack to whatever (since this end will be cut ) e.g. jack to phono.
And since you have some speakers to "donate"/mod then you probably have the jack plug here.

so if you have the jack laying around then here's what you need:
Small AMP
Large AMP

If you want to make it so it takes power from the N900 like mine, well then it gets a little more complicated, look into my battery thread for more info on this.

The amp is very simple to solder, it requires NO external components when running of battery, but if you use an adapter then a small capacitor will be needed, and maybe also some filtering if it is "dirty" power coming of the adapter (noise, low freq switching <20KHz)

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EDIT:
If you just want something to put together fast then you can use the "complete" versions:

Comes in 2x3W, 2x10W, 2x15W

Again the 2x3W can be used from 2.5-5.5V (battery/USB)
The 10W & 15W needs 10-22V (e.g 12V)