Take a good look at the details of how the original top is attached at the header.
Chances are, your original wooden header will need to be replaced; if the wood isn't rotted, the screws will be rusted in the inserts (2 @ each end) and the rest will probably just twist the heads off.
I bought the header (recently) from Moss, but to do it again, I'd try to find one that fits better. Live and learn.
If your top attaches to the header like mine (tacks) try to keep them as low as possible, as I didn't get a couple of them low enough, and show if you look close enough. A magnetic tack hammer is indispensable for this.
Beyond that, all I can add is that you have patience; if the job gets to you, just walk away and come back to it later. You don't want to take on this task with an attitude /bcforum/images/%%GRAEMLIN_URL%%/wink.gif
If you have some specific questions along the way, I may be able to recall how I got past that point, so ask away.