You can use basting glue and an iron to secure the bias strip in place prior to sewing...much easier (and more accurate) than pins. (I use Elmer's Washable School Glue -- much cheaper than the "other" basting glues).

It's also a good idea to stitch down the inside edge of the bias-tape circle first, before the outside edge; that way any fullness can be eased into the outer "stretched" edge. Easing fullness toward the inner edge can create to little pleats.

Thanks everyone for your ideas. My yard stick compass came yesterday and I am anxious to try it.

If you're in a hurry there's always the old string on a pencil method.

I don't think the brand matters, that was just the first one that popped up in my search!

I've used mine for years, and when ever I need a large circle, it's what I reach for!

Karen, if you don't find the yardstick compass at an LQS, I picked mine up at a DIY box store and have seen them at art supply stores too (same brand as Margo showed in her entry).

Margo, I will definitely look for a yardstick compass...I was not aware of them, but I could have used one MANY times. Thanks so much, Karen

The yardstick compass is available in some quilt shops! You can use it on a regular yardstick for perfect circles up to 72"!

http://www.micromark.com/YARDSTICK-COMPASS,6560.html

You might want to pin your background fabric to a tear away or wash away stabilizer. It makes for pretty stitch formation whatever you applique. Stabilizer will also help give the circle a crisp finish.
Blessings,
Jane

I would use a yard stick compas to trace that big circle, and following Beverly's suggestion, on freezer paper (if you have that wide)

Andrée

I would try cutting a 20" circle of freezer paper and iron it to the background. Then position the bias strip on the outside edge, pin (or glue) and sew.

I have just designed a new applique and will need to applique a 1/2 piece of bias in a 20" diameter circle. Does anyone have a good way to mark a circle that big or to insure that the piece is circular when I am done? I am sure there is a technique that I haven't used yet.