I would appreciate a quick run-down on the proper way to frame a door in
an existing load-bearing wall. I do plan to get a permit and have the
work inspected, but I'd like to get a handle on the right way to go
about it.

The wall:

I've verified it's a load-bearing wall by looking in the attic; it runs
perpendicular to the roof trusses. There are no floors above, so the
only load is the roof itself. The next nearest opening from where I want
to put the wall is just over four feet - that is to say, once the new
door is in, there will be 48 inches of wall between the new doorway and
a currently existing-one. There will be no other openings to the end of
the wall (and the house), about six feet going the other way. The
doorway will be for a 30-in. wide interior door. Here's a (horrible
ASCII art) drawing, very roughly:


Existing New 30-in
door |-- 48-in --| door |-- 72 in. (approx) --|
----------[===============]**********[=======================||
|| ||
||++[]++++[]++++[]++++[]++++[]++++[]++||
|| Temporary support ||
|| ||
Non-load-bearing - || House end wall - ||

The non-load-bearing wall above will be about 5 in. or so from where I
want the new door to go in, which I don't *think* will be a problem.
I've also verified there is no electrical or plumbing that needs to be
moved.

I've done some looking around online and as near as I can figure, I'll
need a header across the opening made of 2 2x10 on edge with plywood
between to the width of the framing, supported on cripples placed to the
width of opening I require. What I'm not sure about is how far past the
width of the new opening they have to extend? I would assume they extend
as far as the next uninterrupted stud on each side, but you what they
say about 'assume' - makes an ass of u and me :-D

The related question is how I support the load while doing the work; my
current thought is to put up a couple of 2x6 on edge, nailed/screwed
together, against the ceiling, running from the non-load-bearing wall to
the end wall and supported with 2x4s spaced 16 inches apart, or jack
posts, all resting on a 2x4 flat on the floor. Not sure if the bottom
2x4 is needed, but this work is on the 3rd floor of my house, so the
load had to be transmitted down somehow, and doing it this way it seems
to me I won't need to be quite as concerned about putting the temporary
verticals right over the floor beams. I'm also thinking I'll need to
take the drywall off the ceiling where I put the temporary 2x6 support,
as I can't see drywall taking compression loads too well. Or is that not
a concern?

Anyway, my great appreciation to one and all for any help you can provide.

Yours aye,
W. Underhill

--
"Take sides! Always take sides! You may sometimes be wrong - but the man
who refuses to take sides must *always* be wrong! Heaven save us from
poltroons who fear to make a choice!" R.A. Heinlein, "Double Star"