I'll be replacing my old sliding patio door soon. It is a standard sized
patio door and I'm getting the door from an Andersen retailer. My friend
and I will remove the old door and replace it with the new door.
The new door's RO is approximately 3/4" shorter width-wise than the door
there now.
I'll need to fit it correctly between the studs with shims, etc.
I'd like to purchase all my finishing wood before I start the job but I have
this feeling that in order to see what is really behind the edges of the old
door's track (i.e. wood condition and size/thickness) I'll need to wait
until I pull the old door out.

I am particularly worried about the sill. Are sill widths pretty much the
same or again I should wait until I pull out the door track and see exactly
the width? I need to replace the sill as it is beginning to rot on the
outside edges. The thickness is 1 1/8".

Thanks,
Walter

Don't forget to place pan flashing below the sill to reduce future
water infiltration.

No matter how carefully you plan, there is going to be something
unforseen.
Be prepared to clover the opening with plastic sheeting.

TB