How to make a long straight cut in a board at an angle

Your crown molding is going to have to be special cut. You could do it yourself, however there is a certain pattern that crown mold is cut in to give it its shape (As I am sure you know). One thing to think about in trying to cut it yourself, is that you are going to need a table saw to make your top and bottom cuts. The top and bottom edges are angles either at a 45 degree on both sides, or the more standared 38 degree and 52 degree angles. Without a table saw, you are not going to be able to cut this right, so it will not sit against your wall properly. Also, if you happen to slip you are going to have a noticable spot in your crown molding (and solid wood molding is not cheap).

There are lumber stores that will cut it for you, or you can find one that has this type of crown molding availible for you to order. Just Googole it I am sure it will not be hard to find one that will do it for you, or have the material you need. The machines that are used to cut crown molding get it done with one pass though. the machine also acts as a planer in case there are some thicker spots that need to be evened out. If you need any advise on how to mount your molding look at my answer linked here on How to install crown molding .

Having screwed it together, and as importantly made the holes for the screws, you may need to remake one or more pieces of this frame, or choose new locations for the screws.

In some cases, one can put a clap across a diagonal of a frame to draw it into square if it is not racked too badly. Then new parts of the frame can help to keep it all in the desired shape.

Clamp it up carefully BEFORE you put in those screws. Make sure that everything is in its proper place and stays that way. A large part of doing fine woodworking well is in how much care you take in the small stuff.