It really comes down to technique. The planer stays static, but to get the wood off that you want off is mostly in where you put pressure (right/left, front/back/center) and how you adjust the blade as you go along the timber.

For square rule, you can use a 12" to bring the timber to within 3-1/32 of your lines, depending on how familiar you are with the machine. If you're not fully comfortable with the 12" you can use a 6" or smaller to get it down closer to your lines, or hand plane if needed, leveling it up as you go along. Once satisfied, run that 12" sucker to the lines, a few light passes are best, and with even weight so as not to lose your level face.

Really though, it comes down to your line and your square more than your planer experience. Snap lines are a good thing, and a square that is square is even better.

When you've got a line to work with, you can see where you need to plane to (or hew, in some cases...)
And if your first face is nice and flat and to both lines, you can bring it down easy to your second face with your square square.

Oh, and one tip with the planers, never try to take off more than 1/32 or so in a single pass. The 12" is big, but one still has to be gentle with it. The planer will let you know when it's had too much to eat. They will have settings to allow for up to an eighth or more to come off...I've never gone that deep, but I have come close when dealing with wavy spots.

Use a wire brush to remove as much dirt as possible from the timbers to save your blades. Changing and sharpening these blades aren't much trouble at all. Replacing them is a drag, though, as is not being able to finish a job due to busted blades. 2 sets on hand at all times is good.

My 2 bits.
Peace