Here is possibly the shortest tutorial ever for how to knit through the back loop (tbl).
In the first picture, we are knitting as "normal," with the right hand needle going under the front of the loop on the left needle, and up into the stitch before we wrap the working yarn and knit the stitch.
I apologize to lefties who may be reversing which needle pokes where--this is one reason I put "normal" in quotes!
In the second picture, the right hand needle goes into the center of the loop and under the back of the loop. Then you wrap the working yarn and knit as usual. Er, normal. Er, um...as one frequently does. Knitting-wise.