lol thanks Pawnhead
The best method is to get a 135 degree metal internal angle. The are very much like an enxternal but opposite. Often there is nothing to nail to in the internal so the best method of fixing them is staples. They can be a bit fiddely to get in the right position. If you find the metal is sticking out they can be closed up a little by laying them out on a flat surface and gently treading along them, get a couple of spares cause that takes a little practice.
Forget the adjustable internal tool they are hopeless I won't even try to use one.
To do it without metal angles there are several options however each has there benefits and pitfalls so I find the best way to get a perfect job even though it takes a little longer is this:
Pre fill any gaps and scrape back flat. Then use paper tape to reinforce the joint just like a normal internal. At this stage don't worry if its a bit wobbley. The most important thing is not to have any dry spots under the tape. (check out my video on internals on youtube
and
although these are standard internals the principle is the same.
Now take a chalk line and from top to bottom flick a line so that it takes out any wavy areas. You can either (if you have a good hand) cross trowel using the line to guide you to get a straight corner. Or the sure fire method is to temporary fix an egde to work to following the line and trowel down using the straight edge as a guide. Remove it and do the same on the other side.
I find using furring channel upside down the best straight edge as the leading edge of the "top hat" section is only thin and easy to work to.
I wont go into other methods here because they are also prone to giving you a wavy finish if you are inexperienced and you end up having to use the method descibed above anyway.
Hope this helps.
Cheers
Rod