As people have already stated, try and get a spare panel to practice on, it'll save you tearing your hair out when you've gone through the laquer
When i first started painting, i had the luxury of polishing repairs on 40ft refrigerated trailers with no curves/swage lines!
IMO the curves/swage lines are the easiest to burn through on if you're just learning
As Binman says, keep it lubricated and keep it moving
My polisher is only a cheap 6 speed jobby, but it's ideal for what i need, i usually have mine on setting 2 or 3, but just practice to see what's best for your polish
You'll be surprised how quick you will pick it up