Clean the car to oblivion before you start.
The eye cant see it but cars are covered in crud, the worst usually being tar with tiny bits of metal and other nasties.
If your polishing pad picks them up it can do alot of damage.
A serious detailer would pressure wash and hand wash well before using a special type of clay bar ( i use medium ones by bilthamber )
Using a clay bar is dangerous so watch and read tutorials first.
Use the green pads made by chemical guys and a one step polish like Auto Finesse Tripple All in One.
Any get the polishing pad lube made by chemical guys also to protect the pad and give the polish more working time before it dries up.
Doing one step like this with a DA will only do so much but it will improve the car and would be worth it i think.