Snip three little notches, aka darts, into the edge of the material so that it lays flat onto the bottom of the header, one either side of the heat-weld and another down towards the bound edge. The darts should not extend up behind the lip.
**Notice that I am doing all of this work with the top barely off of the windscreen, just enough to get at where I need to work. I am not folding it so far that it is stressing the windows. If you are worried about this, you can always remove the top from the rear deck and fold it, following the proper factory-approved folding method. It takes longer, but its safe. I have done enough of them that I know not to let the thing get away from me and try to fold up on itself. I’m a professional**