I use sticky notes or even cut pieces of paper...I put like 1-A, 1-B, 1-C, 2-A, 2-B, 2-C etc, and I always pin it to the upper left hand corner of the block...not to the edge, but down in to leave plenty of room for the seams and for pressing. I am not sure how it would be to iron the sticky notes with the glue on them, so I always fold it down and stick it to itself and just pin them to the blocks. I do try to keep my blocks straight, but it is not big deal if they get out of order because I can tell which row, what position in the row and how the block should be oriented if that matters.