Over in the Harry Potter craftalong I posted a tutorial for cabling without a cable needle so I thought I'd share it here too!
Knit to where the cable crossing occurs - this is a left-crossing cable, so the first two stitches we're slipping in the next step cross BEHIND the next two.
Yarn in front, THEN slip the next two stitches to your right hand needle purlwise.
Yarn to back
Knit the next two stitches
Slip left-hand needle into the first two stitches FROM THE BACK
Pull the right hand needle out of all 4 stitches, while gripping the work right beneath the needle to keep the two live stitches from dropping.
Pick up live stitches with right hand needle, making sure not to twist. The stitches are now crossed!
Bring working yarn to back again and now knit the two stitches on the left hand needle (these are the stitches that got slipped at the beginning).
Done! The completed cross.
Knit (or in this case, purl) to your crossing.
Slip the first two stitches to your right hand needle purlwise. Since this is a right-handed crossing, these slipped stitches will cross in front of (rather than behind) the two after. No need to move the working yarn this time, leave it in back.
Knit the next two stitches.
Pick up the slipped stitches with the left-hand needle from the FRONT.
Pull the right hand needle out of all four stitches, again while holding tightly directly underneath the needles so that the live stitches aren't lost.
Pick up the live stitches with the right hand needle, being sure not to twist. The cables are now crossed!
Knit the next two stitches (the ones that were slipped at the beginning).