And of course, click on the pictures to embiggen. Gosh, is that even a word??
For four mats, cut six strips from the width of your fabric, the strips I've used are 2" wide. Sew all strips together using a diagonal seam to make one long binding strip, and press in half, wrong sides together.
Pic 2: Mark the location of the corner seam with a pin. Stitch right up to the pin, and take a couple of back stitches.
Pic 3: Fold the binding strip upwards, and make sure the raw edge of the binding is running parallel to the raw edge of the mat that you'll be sewing onto next. (Just the same as if you're making a 90 degree corner)
Pic 4: Fold the binding strip back down again, pin in place and continue sewing.
Pic 5: Continue this process until you've rounded the second last corner, and stop. Break off the threads, and pin the last corner in place. This is to ensure you have long binding tails to make the final join easier - it's tricky tugging all that mess under the presser foot!
Pic 6: Because I've used a 2" wide binding, I overlap the ends by 2" and cut away the excess binding. If you used 2.5" wide binding, then overlap by 2.5" etc.
Pic 7: Remove all pins, and execute your final diagonal join. I like to pin far enough apart so that my sewing machine foot fits thru the middle without removing the pins.
Pic 8: Now you'll have the exact amount of binding to finish up binding your placemat, and the final join is neat and tidy. You're done.
Just an aside: For small projects like this, I love to be all virtuous and use up lots of small pieces of batting. To save myself the trauma of sewing all the bits together, I use my little friend 'basting spray'. Cut your backing fabric an inch larger than the mats, spray the wrong side of the fabric with basting spray, and arrange the little pieces of batting so that no pieces overlap. Then spray the batting, and place your mat carefully on top. Works every time!