OK, this is my method. As you have seen other people have different ideas.!
Start by putting front and back wings on. This is not the final fit, but is to get the horizontal lines for the doors. Then fit the bare door cases - without any windows or the window frames. Now fit the rear 1/4 panels, getting them as high and rearwards as possible. Now you can line up the doors by fitting shims under the hinges. If these door have not been on this car before, then you may well have to twist the door case (this is called "fettling" and needs a strong knee and a beer belly). When you are fairly happy with the shut of the doors - ie there is a nice even gap between door and base unit where the door rubber seal sits, and the horizontal body lines line up through from the front wing all the way to the back, then try fitting the window frames. If you have got the last bit right they should be able to be adjusted on the door to fit snug up to the gutter and parallel to it throughout. You may need to do some more "fettling" - but don't try adjusting the door again just yet - do all the fettling on the window frame against the fixings to the door.
With luck you now have doors that shut beautifully and with window frames that line up with the gutters. If not go back to the beginning with the window frames loose and re-align the doors. Then try to align the window frames again.
Now you can start on the wings. The front is generally easy, the back generally very difficult. So start with the back. Try to get it as far forward as possible. Feel free to use washers under all the fixing points or none. Don't worry about the bottom of the wing relative to the door. When you have everything else OK that can be eased (ie the strong knee and beer belly again) in or out as required to match exactly the bottom edge of the door. Concentrate on the mid line upwards. When you have the wing accurate to the door, then fit the boot lid and the rear decker panel. Make sure that there is a nice tight gap between the wing and the boot lid. Adjust the rear of the wing in or out with washers to achieve this. Check that there is clearance between the wing and the rear decker.
Now do the other side.
Now you can finally adjust the boot lid and rear decker and fit the lower valance at the rear. This may throw up the need for a few more adjustments, but if the earlier part is done well they should be minor.
Now attach the bonnet loosely. And fit both front wings loosely. Adjust the rear edge of the front wings to precisely match the doors. Now try closing the bonnet. Use the bonnet catch to centre the bonnet at the front. Then adjust the front of the front wings in or out using washers to get a reasonable gap to the bonnet - not too tight. Then at the rear of the bonnet, place washers under the bonnet hinges to raise the bonnet high enough to get clearance to the front wings at the rear. Trial fit the grille to make sure that the front of the front wings are vertical when compared to the edge of the grille (or headlamp surrounds if you are using a series 2 grille).
Last, but not least, fit the front decker panel. It should just about slip in under the rear edge of the bonnet.
All very easy to write, but you are likely to spend up to a week on this. It was by far the most skilled job at the plant when they were made. Regrettably the standard of manufacture of both base unit and panels is nowhere near as accurate as the German norm!
One more small tip; as soon as you have the doors on the car, fit the check straps at the bottom to limit how far the door can swing open. Otherwise you may crease and damage the doors against each other and the front wings!
Hope that helps.
Chris
I'm also trying to retain my patience and achieve decent panel gaps at the moment. Thanks for the advice Chris. I've done almost all of mine now, and in a different order.