I'm not sure I understand what you are asking about 'cornering the trim' but I think you need to miter your corners (whether inside or outside corners). Use a rented miter saw to do the work.
To end the trim you can make an outside miter on the last piece and create a 'return' on it by cutting the opposite outside miter on the end of a small piece and then cut it off square at the back of the miter. This piece can be glued onto the last piece and make it look it grew there.
If you were talking about how to end the laminate floor at another room, you can get a threshold strip to match the flooring, cut it to fit half way under the door or, in the absence of a door you can install it half way of the opening.