How large is it? If it is the pineapple log cabin, I would do stitch in the ditch and have the logs pop up. Or do a 1/4" from the seams on one side of the seam only. In the more busy collage part, I would consider some "thread painting" to enhance that area. You could add any sequins, or other embellishments before you mount it on a frame. I would not use a thick batting because of getting it smooth at the corners. If it is like 30" or so, I would just pin it or use large basting stitches to hold it together. Start in the middle, work say to the right side, then go back to the middle and work to the other side. You would have to turn it top to bottom which would make it easier to work on. Please show it to us when you get it done.