Decide the width of your ruffle.  With fabric folded, cut the fabric strips the needed length.  The rule of thumb for the length needed is generally 2 and 1/2 times the finished length for weightier fabrics and 3 times for light fabrics.

Sew fabric ruffle strips, right sides together and iron open the seams.  Remember, if you do not have a selvaged edge at the end of one of your ruffle ends, you will need to turn under, either at this stage or later when you actually attach it.

With wrong sides together, iron your fabric ruffle.  Then baste stitch a seam the entire length of the cut edge at 2/8 inch.  Make a second basting stitch at 3/8 or 1/2 inch.  You will use this to pull the gathers together.

Pull the basting stitches until it is the length needed.  Baste at 1/2 inch seamline to the front of the garment/pillow, with right sides together.  Then, if making a pillow, sew the back to the front at 5/8 inch seamline, with right sides together, leaving an opening to insert stuffing.

Stuff and handstitch the opening closed.   Have fun...