It seams to be quite easy to do. I got a little interested my self.
Check this video out. You do have to be rooted to do this.
Can i ask why exactly you need this?
Nothing speeds up an old android phone like installing a low profile custom ROM.
You can also install custom kernels optimized for performance to get that little extra.
Slight overclocking is also an option for squeezing even more performance out of it. however that impacts the batterylife.