I make these sort of quilts all the time, as a colour wash. When you are making the tube, make sure that you start pinning from the centre out to the edges. This stops the tube from twisting. Before you make the tube, it is also a good idea to iron each alternate strip so that the seams go in different directions. That means that you have one strip with both seams pointing inwardly and the next strip has the seams pointing outwardly. Hope these tips help you to have better success, as mine always go together quite easily with this method.