The walking foot would be my choice. It keeps the layers moving at the same rate and averts puckers. When you SID, align the foot so the needle runs along the LOW side of the seam. (that is the ditch) Go slow and hold one hand on each side of the seam so you can help guide the seam (pull the seam gently apart so you can see the seam better). When you get to a corner where the low side is opposite the side you are sewing on, simply sew to the corner and guide the needle to the other side. Continue sewing in the ditch. Hope this helps (and makes sense).