For me, the following things would have been great to know roughly 2-3 years earlier:
Sewing: you can set in a simple 1-seam sleeve before sewing the side seam of the garment or the sleeve and you can sew in a zipper before sewing up the rest of the garment. Really. The universe won't implode because you did not follow the step-by-step instructions in the right order. HOWEVER when they say 'copy all markings' they definitely mean ALL markings!
Knitting: you really, really should learn about fiber properties before making yarn substitutions (and NO, alpaca is NOT 'practically the same as wool').
We make these mistakes so that others don't have to
