1.split e deck into 2 halves.place them end to end as though u about to do a normal shuffle.keep track which half was originally e top half.
(suppose u wan keep e top card e top card)

2.start riffling e ends by releasing cards with ur thumbs.let e final few cards fall frm e top of e deck.to retain bottom card,do e reverse.

pretty easy eh? by timing e riffling u can control which cards drop lst or first. try it.its simple

good luck n all e best in magic