Can someone explain to me how I can get a straight line when painting into a corner of a different color?

Not sure how pro`s do it but what I do is hold a piece of rigid plastic in my left hand -- brush in right -- move plastic down wall A as I move brush down wall B -- Savage

I would mask over the other color dried paint using the kind of masking tape that is an easy release and is absolutely flat, i.e. does not have any of the crinkles that less expensive masking tapes have. Paint tends to "bleed" under the crinkles no matter how hard you think you have pressed it down. A one-inch wide tape should suffice.

Using a 1 inch wide finely tapered, angled bristle brush I would then cut-in using a minimal amount of paint, but enough to cover the old paint, using light brush strokes up to the masking tape, not painting in a straight line but like I am making a lot of small "x's". Cut-in about 2 inches out from the tape. If you paint in a straight line using the brush, and wait too long before using the roller, then you will more than likely see your line that you have made when using the brush. Ideally, painting walls is job to be done by two people - one to do the cutting-in and the other to do the rolling.