First up is the simplest and most basic ruffle. It is just a gentle ruffle - nothing too frilly or over the top.
All you have to do is cast on twice as many stitches as you plan to use for your main pattern. Knit a few rows - I like the look of a stockinette ruffle, but you can certainly use garter stitch if you like; or experiment with different stitches such as seed stitch. After a few rows, you will need to decrease. Just work a row of K2tog - or P2tog even. So for instance, if you plan to work with 10 stitches and you've cast on 20, you will be removing 10 stitches on the decrease row and be left with the 10 you need for your project.
Next is a more ruffly ruffle. This one simply expands on the above idea. Cast on three times as many stitches as you plan to use for your main pattern, work a few rows, then decrease. You can simply work a row of K3tog or P3tog as the decrease, or you can flex your knitting brain and calculate how to work the decreases over the next two rows instead of just one. Knitting or purling 3 together can be a little tight/tricky, so you may opt for spreading the decreases out over two rows. For instance, let's again say you want to work with 10 stitches. You have 30. You could work the decreases this way: Next row, *K1, K2tog* along the row. This should leave you with 20 stitches. Next row, K2tog across the row, and you should have left the 10 stitches for your main pattern.
Finally, you can pick up and knit along the edge of a finished piece to make the ruffle. You want to add the ruffle to a finished edge, not live stitches. Ruffles need a nice, firm edge to hold on to. If you add a ruffle to live stitches, the ruffle will spread the fabric out.
Say you are knitting a scarf and want to add a ruffle to each end after the fact. To make the picking up easier, you may want to use the crochet cast on so that the cast on stitches are clearly defined and easy to pick up. Your standard knitted bind off also created a nice edge easy for picking up and knitting. I'm sure there are other cast ons and bind offs that will give you a nice edge to pick up, but since I typically just use the ho-hum knitted bind off and crochet cast on or long tail, I couldn't give an example of any others.
In this particular sample, I picked up 14 stitches along my cast on edge, then knit a few rows in stockinette, then increased big time. I knit into the front and back of each stitch in the next row, as well as making a yarn over between each stitch, totaling 42 stitches. I next worked a plain row and then bound off. This gave me a very ruffly, eyelet edge.
You can knit the ruffle as long as you like, but the longer it is, it will turn to a flounce as opposed to a ruffle. It's really all a matter of personal taste. So get out your yarn and needles and start playing. You might like the same method of making a ruffle in one type of yarn but not another. Experimenting is part of the fun of knitting.
Here is a video of the ruffles in this post. Happy Knitting!