You're getting close! Your t-shirt quilt is starting to look like a quilt now. Are you getting excited?
Okay, now I do a quick iron (again). I already ironed the blocks before piecing them, so really all I'm ironing is the seams between the blocks. I want them to lay nicely together.
Then, I lay out my backing on my guest bed. (You can use the floor, a large table, etc. Just make sure it's nice and flat.)
For backing, I like to use a flat sheet.
Smooth it out really well, pulling out the wrinkles. Then, lay the batting on top.
I always use 100% cotton batting.
Then, you want to pin, pin, pin...
I use basting pins, because they are curved, which makes it easier to pin through the three layers.
This part of the process is called "basting" and there are lots of different techniques (spray adhesive, for instance). Just google "quilt basting" and you will have tons of tutorials to choose from. Find the one that works best for you. :)
Next step: Quilting