Kudos! Good job - the 'arm' looks great.
Isn't starch *wonderful*! Another tip - you can glue baste the seams together instead of pinning - put a wee drop at the intersection where the seams will meet (at the 1/4"), press and sew! This is using Elmer's School Glue - which is washable. There are special bottles that give you a nice teeny drop or thin line (craft/paint stores might carry them).
Your fabrics are gorgeous - this is going to be a stunning quilt!