I audition designs using sliced apart sandwich bags & a regular marker.
I mark my quilt with Crayola Ultra Clean markers. At some point I hope to get good enough at FMQ not to need to mark but for now I am focusing on training myself to learn good habits: consistent spacing, continuous line, consistent sizing, filling the entire space. By marking my quilt, I am able to focus all my active concentration on properly formed stitches.
I make sure to pick a color that is very different from the quilt top/thread color (those two nearly match for me since I want to hide my quilting). For example, on a cream/pastel quilt top with a pastel variegated thread, I would use a red or purple marker because they will show up the best against the background and my stitches will be very visible against the marked line. The spaces where I have small red squares (I have 4), I will use a blue marker to mark my line so it shows up. I can switch colors as desired since they all wash out when I'm done (I soak my quilt in the tub for 15 minutes with a Shout Color Catcher; drain the water; re-run the water with a tiny amount of Tide Free & hand agitate; rinse & then transfer to my washing machine to wash on the gentlest cycle; once I've verified there are no stray marks, I transfer to the dryer & dry on low).
Hope that helps! I know the ideal is to be able to FMQ with no drawn lines, but I know personally all that did was make me frustrated as my lines were all different sizes with varied spacing. I was spending more than half my time with the seam ripper. Now my motto is to mark it, quilt it & enjoy it! I only rip out a line of quilting if the stitches don't form, are pinpoint tiny, or are very unbalanced. Other than maybe twice over the entire quilt, I leave the seam ripper in the drawer where it belongs.
Good luck & have fun!!!