kernel-headers & compiling source

[Jeff Penn <jeff@jrpenn.demon.co.uk>]

I have read through the kernel-header docs, & am still not sure I 
understand what they are for.  I assumed that they enable source to be 
compiled when using a kernel-image.

If this is correct, what is the procedure for compiling i2c-source or 
lm-sensors-source?.

thanks
Jeff