I always just kind of wing it when I make t-shirt quilts. I find it easier to work on the floor than my design wall. I rarely add the narrow sashing. I usually mark out on the floor the size with painters tape. I interface and cut out the larger designs and lay them out in a way that I like. I cut them a little bit oversized. I then do the same with the medium sized designs. I go to the smaller designs last and cut those to fill in the holes. I trim the pieces to make it fit. Hope this makes sense.
I am not a plan ahead type of person. I usually plan as I go along on most of my quilts.