You do not have to cut from lengthwise grain. You can use rosswise without a problem. Either way, you will have to piece your border. I like to piece my border so that each side has the seam at a different place (one side towards the top and the other side towards the bottom). Not necessary, but I think it is less obvious (could just be me!!!). If it is a print fabric, try to blend in the seam to the print. No one will know! The most important thing is to not stress, keep it a fun project or it will become a UFO - we have all done that!