Use the lightest weight fusible stablizer you can find. Making sure that you put the stretch of the stablizer crosswise of the stretch of the t shirt. I cut the stabilizer into pieces slightly larger than I want, fuse it to the shirt and then cut the block to size.
Use a wee touch of a washable glue stick to help hold your seams open rather than pressing to one side (less bulk). Press nicely but remember not to press over any rubberized, painted, etc. emblems on the t shirt.
They look good with or without sashing. This picture will show some of the blocks I am working on. They are not yet in final placement and do not show the sashing.
This is not the final placement, just keep the blocks on the design wall as I am working.