Now it's time to make the quilt sandwich!
Depending on the width of your solid fabric, you may find your quilt back is several inches taller than it needs to be. To preserve the proper alignment, match the bottoms of your quilt top and back, as shown above. (In the photo, the left side is the top and the right side is the bottom.) This will help to ensure that the pieced strip with the letters sits slightly lower on the quilt back.
As you smooth the layers of your sandwich, make sure the seams on your quilt back are parallel to the seams on your quilt top. The will help to ensure that the row of letters on the back of your quilt ends up being staight.
The number of pins you use for basting is really up to you. You want to have enough pins to keep your sandwich secure, but remember that you're going to have to stop and remove every pin you put in. If you're going to be free-motion quilting with a darning foot, I think that placing pins about 3 squares apart (so 3 squares between each pin, which ends up being about 6" apart) is plenty.
As always, I recommend placing your pins in some kind of regular grid arrangement, so you know when to expect to encounter them during quilting.
I would like to challenge all of you who are making this quilt to finish it with a new-to-you quilting design. My recommendation is a boxy meandering stitch that mimics the shape of the subway "lines" on the quilt top.
Start by practicing drawing the pattern on paper. This design is conceptually similar to a standard meandering stitch but, instead of making a continuous curving/squiggly line, you'll be making a continuous boxy line.
I recommend practicing on a scrap sandwich before moving on to your actual quilt. Start in the same place you would normally start for free-motion meandering (for me, that's in a corner) but, rather than moving your sandwich in a smooth curving motion, move it back and forth and from side to side to make boxy shapes. You may find that some of your quilted "boxes" are a bit rounded on the edges, which is totally fine.
You'll probably notice that, much like with curved meandering, making quick jerking motions can result in too-long stitches, but failing to move can result in a pile of too-tiny ones. Practice until you find a rhythm that works for you. I find that the motions I make while doing this stitch are kind of robotic (up and down and back and forth, etc.). In fact, it may help to think about this as the quilting version of doing the robot, or even to listen to dance music while you're quilting.