Firs i level the head of the door and shim bottom to make level.
Then If the side frame jambs are close to plumb I center the top with shims and nail.
Then i adjust bottom side to side to get the reveal at the top of the door perfect. Shim and nail.
Exterior door you can hide a screw or nail behind the weather stripping.
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The most important one when first installing is the bottom.
If it's off the whole door will be off. The whole door will rack causing all the exposures to be off.
How far off it it?
Just lay a 2' level on the floor and lift it untill it's level and take a measurement.
I hate to see someone try to use shims under a threshold. Reasons being, then the threshold is not making 100% contact with the subfloor it will flex bend or twist when steped on causing the seals on the ends to crack and leak.
It leaves gaps on the outside where water can get in.