Garage door opening preparation is critical for correct garage door installation.
Garage door sizes are always shown and listed as the width first and then the height.
On the inside of the garage your opening should be framed with wood jambs, 2″ x 4″, or 2″ x 6″ if possible.
The side jambs should extend to approximately the same height as the headroom required
If you just removed an old door, the jambs should be inspected for the condition of the wood.
If the wood is rotten, it should be replaced now.
The jambs should be plumb and the header level.
If there are any bolts fastening the jambs to the wall, the heads should be flush, so they don’t interfere with the installation of your new door.