Well I've tried this out on my spare yarn/needles here. This is what I did.
I knit into the back and front of the stitch then took the stitch off the needle.
Then, If you look at those two stitches very carefully, pulling them apart on the needle, you can see a strand of yarn running vertically. Stick your left needle into that strand and knit into the back of it.
It would be too tight to knit into the front of it, as you'll see when you try it. The trick is to look closely at the two 'made' stitches.
If you are still having trouble I can take some pictures when I get home from work. Unless someone beats me to it in the meantime!