When hanging an exterior door, how can get the door to close evenly top & bottom. The door is plumb inside the casing, but the top of the door is not meeting the weather stripping, while the bottom is. Thanks,

Not too plumb.

I take it that you have a gap between the vertical edge of the door and the door stop.

One or both of the legs (sides) of the frame is not plumb.

Put your level on the edge of hinge side jam. Make sure that it is plumb that way.
Put you level on flat of jam and make sure that jam is plumb that way, shim if necessary.
Temporarily secure it top and bottom.

Plumb latch side, temporarily secure.
Close door, adjust latch side frame to fit door if necessary. Shouldn't take much adjustment.

Shim frame to achieve consistent gap between door and frame, top to bottom

If you mean that there is a gap along the top, the door frame is not in the opening level. Raise hinge side or lower latch side. Should be 1/8" gap between the top of door and head jam.