hi,
if you dont know the mechanical means of lowering a car, then you
should seek a professional to do it for you, as they have the
proper tools and can do it within 2 hrs.
its a good thing to change your shocks if you change your springs.
using stock shocks can wear out easier and 'bottom out' which isnt
really a good thing.
if the springs lower your car more than say 60mm, prepare to have
scrapping, esp speed bumps! or driveways.
goodluck