When working yarnovers all you are doing is wrapping the yarn around the needle to create a 'hole'. This increases one stitch in your work. For lace you combine those yarnovers with decreases for decorative openwork that keeps the same stitch count. For a standard yarnover you wrap the yarn around the needle in the same direction as you knit the stitch. For a backwards yarnover (creates a smaller hole) you wrap it in the other direction. When you are working a yarnover between different combinations of stitches (knit and purl) it works a little differently. This video goes through all the possible stitch combinations.