Yes its easy to use koi science nai lagane parte is ko use karna main
, just go through the instructions and c a video on youtube before using and everything will b clear to u
Clary bar is used to remove contaminants like over spraying, industrial pollution, brake dust etc etc
For more clarification have a look and the images , the 1st one is after wash and the 2nd one is after using clay bar 
<img src="//fcache1.pakwheels.com/original/4X/8/3/5/835e3eecacd0ba0eeac4bf012364361d1bade631.jpg" width="250" height="277"><br/>

2 bucket method is necessary if u do not want swirls on ur car
as far as the dust , dark colors main yea hi masla ha , u can use a detailer spray for that 