I haven't lowered my 306 but someone elses, a friends, I used a video off you tube it's pretty straight forward,
But that's assuming that you know how to change the front springs
Rear is torsion bar setup, I would be inclined to upgrade to the rear beam from the Gti6/s16/xsi/hdi as it uses rear discs and has a stiffer anti roll bar, but you don't have to it is just a nice upgrade, I'll post the videos I used to lower the 306 in a thread on the 306 section