If I was doing it I would quilt it on the diagonals both ways through the center of the squares. You should be able to do that without having to mark it. If you baste it really well you can do this with continuous lines by “bouncing” off the side. In other words, when you go across and get to the edge, you continue on at a right angle until you hit the next edge, then do the same thing. I did a lap size quilt this way and did not have to break my stitching once. Hope I have explained this so it is understandable. Love your quilt!