Apologies for the pic--I accidentally knit these ones extra extra extra lusty so don't have a model for them!
This pattern is my general tried-and-true, not the one featured in the pic.
Using size 13 needles, I CO 34 sts of three strands light weight acrylic (medium--Use 32 sts for S or 36 for M+).
K1, p1 every row until you achieve a desired length or close to it (your length must be knee height or shorter or you'll have to add sts). I keep a stitch counter going so I know exactly how many sts for the next warmer.
When the leggies are nearly as long as I like them to be, I knit all sts all rows for 6 rows. This helps to keep a natural elastic around the knees. You should have a huge knit rectangle/square. Sew together. The CO row goes near the ankles. Repeat.
Feel free to weave ribbons in and out of the tops or sew on buttons/zippers. I've sewed zippers into the seams rather than attaching them directly, too.