Welcome back to the Vintage Sheet's Octagons Quilt, if you haven't seen what has been going on so far you should have a look here, here, here and here. This is the quickest stage of making a quilt so I will be coming back in only one week writing about the quilting. The quilt sandwich is the name we use for the layers of a quilt that come together, normally a patchwork top, wadding (or batting) and a quilt back. My quilt back is simply one very large vintage flat sheet but if you can't find one piece of fabric large enough you can make a patchwork back, or sew a few pieces together. If you decide to sew a few different bits of fabric together its a good idea for the left to be a mirror of the right, this makes it obvious its the back. Also avoid having a line of stitching straight down the middle as this is where you will probably fold it and it will be less likely to life flat again afterwards if you stitch here.
So once you have your three layers you simply need to lie them down in the right order. Start with the back and lay it face down on your work surface (I use the floor). Then you lay your wadding (batting) over this and spread it out as evenly as you can. It's quite tricky to get rid of all the bumps because it gathers up when its rolled into the packet to sell so just do your best. Then lay your quilt top, the patchworked section, over the top making sure it doesn't go over the edges of the other two layers and if possible your backing and wadding should be larger than the quilt top by a few inches all the way around.
Then roll up the other side so that it looks like the above.