To crochet a ruffle, increase the number of stitches per row. Increasing by at least one stitch every other space makes a row too wide to lay flat and creates a ruffle. Gathering or layering sections can also create ruffled looks.
The simplest way to make a ruffle is to continue from a pattern with a row of single crochet matching the width of the previous row, then turn and make two single crochets in each stitch. You can add ruffles anywhere on the project by tying on and slip-stitching or single crocheting into the spaces of the existing stitches. This is easiest on the borders, but is also possible in the wiggly crochet style across a scarf.
To make different types of ruffles, you can adjust the stitch and frequency of the increases. Fillet crochet or fan patterns can make a lacy ruffle, so long as each row increases evenly.
To add a gathered ruffle, start by crocheting a few rows or cutting fabric, into a strip two or three times as wide as the desired finished ruffle. Take a needle and thread or tapestry needle and yarn, put a sturdy knot in the end, and put a loose running stitch lengthwise through the strip. Pull the unknotted end, and gather the fabric along the thread. Then sew the gathered edge in place on the scarf.