RhondaK ... the best way to figure this out is to take the idea and make something small, just to try it out. Each time I do it, I use a scrap piece of fabric, just to check what width I want before I go to the "real" fabric, as it takes a lot ... and if you miscut, you can't glue them back together!
Some notes for you ...
Your seam line/allowance is whatever you want your finished binding to be.
Thus, you need to have it trimmed to allow for that.
For sure, there should be batting and backing out to the edge.
Sometimes, I have top fabric right to the edge.
Other times, there will only be the 1/4" and then the batting beyond. That would be in the case where I have star points that I don't want lopped off!
That also works when I have something narrow, but want it to be wider. An example would be this runner. It was very shy on width, as I was working with a narrow 11-1/2" strip of leftover fabric from another project. By doing it this way, I got a runner that is 13" wide that works perfectly for a small end table. The binding finishes at one inch.
Otherwise, just stitch your binding on the same as you would for a regular binding ... with the wider seam allowance and binding the full width of that
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