Arrange a builder’s level and use it to verify that both sides of the door are plumb. Measure all sides of the door, and mark mid points on the sides and use Set Square to verify that that all sides are parallel. Check faces of the studs on the wall on each side to verify they are perpendicular to their adjacent walls. If Trimmers are not perfectly aligned, use a pry bar to move it either a bit up or down till it is perfectly aligned with the door. Put shims in between the wall and the trimmer, and then use nails to fix shims in their place.
Use the level on the base of the door to check if it’s perfectly aligned or not. If either side is a bit lower than the other one, use shims under them to put them in the proper place, and then adjust the screws on the side of the panel till both sides are on level with each other. Also use a pair of shims to keep the vertical section level as well. If there is any gap in the opening point, adjust a shim in between it for now. Fix all these shims with small finish nails.
Use a helper to lift the door into the opening. Place the sill on the floor, or shims if you have fixed them underneath your door. Move the door panel in order to make opening between the door and the wall frame. The spacing should be equal on both sides.
Place shims between the hinge and the jamb leaving enough space for the trimmer. Fix the jamb to the trimmer with the help of nails. Place shims in order to fix the jamb at the proper place. Check out that if the door is plumb and the jamb is aligned with its adjacent wall surface.
Secure the hinge side of the door by inserting shims at the centre point of the hinges or where ever necessary to keep a fixed space between the door and the jamb at all places.