Salvage, not sewage

So after sewing my last set of strips, cutting out all my triangles, setting it out, making the pattern how I liked, I sewed the triangles together to make squares. I pressed all the squares, sewed them together to make the full quilt. Now it's time to figure out how to cut off the salvage, sew the 2 large backing pieces together to make a huge backing and then make a quilt sandwich.

To me, this was gibberish about a week ago. But I went to trusty quiltfabric.com and asked some questions. I bought the fabric I needed (2 pieces that are 1 and 1/2 yards each). I need to sew those 2 pieces together across and that will make my large backing for the whole quilt. I also need to cut off one edge (the salvage), before I sew the pieces together...which edge? Not sure.

I bought the batting (the part that makes it a soft and cozy =) and a spray adhesive, along with a pair of gloves for free stitching (the part that makes it look quilty), they're basically gloves with fabric thimbles at the tips of the fingers.

Apparently the part I have done so far is called piecing, whereas the actual quilting is the free stitching, which THANKFULLY you can do with a machine now. (Otherwise, I'd be stitching loopy stitches all over the quilt.) So I'm good to go, except for the salvage part. I thought I could figure it out with Google, but I'm stumped so far. Will continue searching...