For yo's with purls you would do the opposite that you do with yo's with knits
With knits you bring the yarn to the front (like you were gonna purl the next stitch. And when you KNIT the stitch the yarn just goes over the top of the needle, you knit the next st and you have your yarn over.
With purls you would take the yarn to the back, (like yer gonna knit) insert the needle to purl and bring the yarn OVER the needle to the front and when yer done purling that st you have a yo.
Hopefully that makes sense.