You don't actually slip the stitches from one needle to the other.
Once they are evenly distributed (ok so this requires some slipping) make sure that they aren't twisted and get your spare needle.
find the first stitch cast on and using the tail which should be joined at the other end knit using the spare needle, which will join them in a circle.
now use this needle to knit (or purl etc as required) all the stitches of needle 1.
Once you have knitted (etc) all those stitches needle 1 is free! the spare needle is attached and needle 1 goes one to knit (etc) all the stitches of needle 2
and so on.
Was that more confusing? I'm sorry if it was.
It easier to give it a go.
That's with DPNs of course, knitting with a circular needle is far easier - cast on, don't turn, just find that first stitch you cast on and keep going...around...an
d around...and around.
Oh and continuously knitting (without purling) produces stockingette stitch
Soooooooooooooo I am going to use circs. I will post here later with an update or more questions!