Bring the yarn you're wanting to trap between your needles in the opposite direction to if you were doing a knit stitch.
Bring the yarn you're wanting to continue knitting with between your needles, as you usually would to begin a knit stitch.
You then need to complete this knit stitch without knitting the strand of yarn you're needing to trap. You can take the yarn you're trapping off the needle to make it easier, as it should already be held in place by the other yarn.
The float should now have been trapped so you can simply continue knitting.