Using 4mm circular needles, cast on approximately 100 stitches (I am a size Small (US)).
Join, being careful not to twist your work.
Purl 5 rows, knit 5 rows to get a "ribbed" effect.
After approximately 40 rows, begin decreasing by knitting two stitches together every 10 stitches on every third row of purled stitches.
Continue this way until you have reached your desired length (I'd recommend knitting a bit longer than you think you should)
Cast off VERY loosely (it is important to use a stretchy cast-off for these)
Seam the bottom (about 5 cm) in the center to create a crotch.