The above instruction is good. My opinion is my opinion so take it for what you will....
Leveling to the gun is a good starting point and not the be all end all. What you care about for long range shooting, especially if you are dialing your elevation corrections, is that your reticle is true when you shoot. Whether your gun is level when you pull the trigger is pretty irrelevant, the bullet always falls straight to the earth. Doesn't matter if the gun is upside down, sideways, or perfectly level.
What type of shooting do you do? Field or bench?
If bench, leveling to the gun is probably good. If you're in the field most of the time my recommendation is the following...
I'm mainly a field shooter. Other bench shooters could probably advise you better.
Find a level you like that can be used without breaking cheek weld. My recommendation is the Darrel Holland level. It works kinda like picture in picture use to on the old TV's. Your non-dominant eye keeps track on it while your aiming. The NF top-half system is garbage from my experience.
Believe it or not your body memory is pretty good. I took a digital angle meter and did some testing one day with a shooting buddy. My body could repeat the same comfortable shooting position and cheek weld without an indicator to within 0.2 of a degree. The NF had a tolerance on their bubble of only 0.8. I can't remember what the Holland was but it didn't matter because I can see if its perfect without breaking cheek weld.
So my advice is this. If you can afford and don't mind a level hanging on the side of your scope get one. It is the most bullet proof way to go. And get a Holland. Then mount the scope so that the reticle is level when the gun is shouldered to your most repeatable and comfortable position.
If you don't want a level, mount the scope so that the reticle is level when you and the gun are in your most comfortable/repeatable position and rock on. From my experience, you body memory is pretty good.
Using a plumb bob hanging from a tree limb is the best way to go for all of this in checking level of the reticle. Get a shooting buddy to help, it'll get the job done better.
Good luck.