I have found that if the garment has been cut with enough 'room' then I can usually get enough for an 8.5" square that I can use for background pieces for most of the blocks.
Sometimes I may want to use fabric from the arm or leg of a garment and in this case I find cutting 4.5" squares and making them into 4 patch blocks can provide background fabric with just a little more interest.
The top right hand block has a four patch background.
The bottom left is from a newborn stretch and grow - a lovely outfit - I could not put all of the garment on the quilt so I selected a part which reflected what it was like.
These images are all from the creation process. I placed them on the design board as I proceeded but lots of changes still to be made to come to the final get together of all the pieces.