I started by ironing heat'n bond lite on the back of a bunch of fabric squares (by Basic Grey). Then I die cut balloons, happy, birthday and the star border (all from PTI).
I cut a rectangle on the fold of some muslin fabric (so I wouldn't have to see the bottom). Before sewing the bag, I ironed on all the die cuts and ran stitches over them and for balloon strings.
Have a great weekend!