The exact location varies depending on the door.
How to attach a garage door arm to the chain.
Pull out the cotter bolt with the pliers.
Swivel the arm down and allow it to.
Attach the bracket that connects the arm to the door then attach the arm itself.
You should have 1 2 inch of play in the chain.
On the chain there is a device called the trolley.
This is where the opener arm attaches the opener to the door.
Using pliers remove the cotter pin from the cotter bolt that attaches the upper end of the door arm to the carriage.
Press down on the chain with your finger.
Grasp the chain 1 foot from where it connects to the end of the threaded chain anchor.
There is a threaded eye bolt that can be adjusted to get enough slack to attach the chain.