You already know that I quilted 9 x 17" blocks (blue) just to try this joining lark. I wanted 2" sashing.... (this method would work for any size. You want 12" wide sashing? he he he... OK, I exaggerate but it will work). ..
So what I did was... cut & sandwiched a 2 1/2" sashing strip (red) and quilted it, in this case I used just one straight line of walking foot quilting. I think if you wanted to do some free motion, it would be safer to cut the fabric wider than the desired sashing so that it could be trimmed to the exact size after quilting. Then I joined the red, quilted sashings to my blocks with white fabric using the same method as for Quilt-As-You-Go, easy!
I used white fabric for my joining strips for two reasons... 1. I wanted it to show up for demonstration purposes, I could easily have used the red fabric so that the join wouldn't be so noticeable. 2. I think it looks good!
Considering this quilt was only an experiment.. I am very much liking how it's turning out you know. Today I "would like" to add borders... more later.
You can see how many times I wrote the word 'sashing' almost every post has the blooming word... wouldn't you think that the spell check might have realised that it should add "SASHING" to its word bank! ha ha ha