First thing is to get the old roof off and back to the original channel, if it's still there... Once the old is removed, set your new roof on and slide it back and forth checking to see where it matches the contour of the car best.

If the original channel is still there, then I start by using some 1/8" X 1 1/2" strapsbent to the contour you want the roof to follow. I'd suggest one of these straps going from side to side place about every 18".

Mark the new roof from the inside to get the correct opening, then make your initial cut about 3" outside of this line. I use Cleco's to get the roof installed in the same position every time I reinstall it, but self tapping screws will do the same thing. If the contour is not quite correct, some relief cuts with a sheet metal shear will make the roof easier to make match the contour.

Slow and patient is the key... Don't make any final cuts until the contour is correct and the crown is the way you want it.

When the final fitting is done, don't try to weld any beads, just stitch the roof into place, with welds starting about 4" apart. let the weld cool. Work the area around the weld to get the sections to match up. Take it slow, hurrying on the install will cause you nothing but grief and lots of extra labor when it's time to start finishing the surface!!!!

You'll probably have some more specific questions as you go, don't hesitate to ask!!!