I always remind myself that knit stitches ALWAYS have the yarn in the back while purl stitches ALWAYS have the yarn in the front.
So first I make sure my yarn is positioned correctly.
Then the only thing that changes in a thru-the-back-loop stitch is the direction you insert your working needle.
In a normal knit stitch (yarn in back) you insert your working needle from left-to-right. A thru-the-back-loop means insert the needle from right-to-left, snag the yarn at the back and pull it forward thru the loop.
In a normal purl stitch (yarn in front) you insert your working needle from right-to-left. A thru-the-back-loop means insert the needle from left-to-right, snag the yarn in the front and pull it back thru the loop.
The tricky thing in the purl-thru-the-back-loop is the awkwardness of the angles. You have to start from behind the needle holding your stitches in order to get the needle to pass from right-to-left and then come forward in order to snag the yarn. I usually have to twist my wrists a bit to get it to happen.