How to Fix A Door That Slams
From DIYinfo.org
The only solution is to fit a surface mounted hydraulic door closer.
They are often supplied with a paper template which acts as a guide for the fixing holes to be marked at the top of the door and on the architrave above.
Method
1 If a template is not supplied, hold the door closer in position on the top edge of the door on the hinge side, and use a long bradawl to mark the screw positions.
2 Drill pilot holes, and screw on the closer. Chisel out a flat recess for the pivot arm fixing plate in the architrave, and screw it on.
3 Fix the pivot arm to the body of the closer, and turn the adjusting screws on the closer so that the door closes smoothly and slowly without slamming.