Question
How do you do slip 1 purlwise through back loop?
Answer
Put the right needle through the back loop of the stitch from left to right and slip the st off the left needle. This will cause the st to twist.
I found video showing how to purl through the back loop. This should help a lot in term of where to place right needle. When you do your slip stitch, you just slip it off without purling.
Here is a link to the video;
How To Purl Through The Back Loop
Here is another video showing how to slip 1 though back loop. It might not be too clear but it helps. Although, it doesn't say sl 1 purlwise through back loop but that is how we do it purlwise.
Here is the a to How To TBL;
How To Slip 1 TBL (through back loop)
Happy Knitting!
Ratcha