Step 1 closing the garage door before beginning use the garage door opener to close the door and fasten it.
Tighten garage door belt.
Tighten the adjuster three full turns clockwise to add tension if the door closed and did not remain open halfway.
The specific procedure should be in the manual for your unit.
The belt needs to be connected with the tensioner loosened so that the tensioner can be used to take up any excess slack.
The manual does not indicate how to solve this problem.
All you need to get started is a step ladder a 7 16 inch wrench and a flat head screwdriver.
The only thing you need to do with a belt driven opener is to adjust the belt occasionally.
But basically assuming the belt is attached with enough slack removed the tensioner bolt should be able to taske up the rest of the slack once tightened.
In most cases all you have to do is adjust the opener belt of the belt drive tightening it to ensure smooth running.
Brand new chamberlain garage door opener with excessive slack in belt drive.