I agree with all that, for the most part, but considering that this is a garage and we
don't know how it was constructed, the OP should be prepared for surprises when
it comes to the framing. I've seen some real hack work done when a garage is
DIY (or even contractor) built.
I suggested checking out some YouTube videos to get a feel for how a door install
is *supposed* to be done and then taking a look at how the actual framing was done
and see if things match up. Be prepared to modify some of the standard steps.
As far as measuring the rough opening, one key step was left out:
- Measure the width of the rough opening in 3 places. Top, middle and bottom.
- Measure the height of the rough opening in 3 places. Left, right and center.
- In *most* cases the smallest measurement of each set of 3 is the one you want
to use. In some cases, the current framing may be such that the smallest measurement
is just below a standard size and the installer may want to consider taking steps to
widen/lengthen that area so that a standard size door fits. BTDT
In some cases, such as with my basement door, a standard size door just won't fit.
The rough opening height called for a 76 1/2" door. The price difference between a stock
door and a custom door was beyond ridiculous, especially for a simple flat panel steel
door. I bought a stock 80" steel door and cut down the jamb and door to the exact height I
needed. This helped.
https://www.lowes.com/pd/DEWALT-7-in-Turbo-High-Performance-Aluminum-Oxide-Circular-Saw-Blade/1100901