Ive been prescribed 10mg tablet Dexies for quite a while, most peoples I ask find that 20mg is a good recreational starting dose.
orally:
Wait a good hour and a half for full onset effects, you'll probably have to poop too. Then pop more when you feel its time, youll know when its time. You may hit zombie status if you over do it. Zombie status isn't cool.
Nasally:
Prepare your nose for enough powder to choke a camel, crush two pills very finely, then waste the dexie

I have found that many people who snort dexedrine get less euphoria, and a longer lasting jittery-cant-sleep-hate-life "buzz". Not so sweet.
i.v.:
yeah, bad, deadly, etc. filter filter filter, let sit then filter again. Use a good amount of water, and very large needles. Did I say filter? Anywho, ~25-30mg iv'd is comparable to iv'd meth, or so my meth booting friend said. I'd recommend not doing this more than once, and only after you can find a good way to filter the snot out of those suckers. Youll probably sweat a lot, feel all hot and rushy, then be "chill/cool" for a decent while