I think that everybody has their own way of teaching things- for example, the story goes that Goldman taught Marciano to throw an uppercut using towels draped over the back of bar stools- but I have never heard the piece of paper routine described. Wait and see what he tells you on Thursday.
There are a few things that go into a good, straight left hand (jab). You need to push off your back foot, not lean onto your front foot. There is a very slight turn of the hips, accomplished without shifting the weight on the feet, and a small turn of the shoulders. You need to drive the arm into full extension, and rotate your fist as you do so. And the timing of all this has to be perfect.