How I learned them was, I learned lincons got decent at them. Then I would go off the jump and concentrate on throwing 90 then linocon then 90, took me a few tries (I found the last 90 is hard and frequently used to have to butter out) but I got it in a pretty short amount of time.
try setting it like a backflip, but look at the lip when you take off. i dunno if that helps but thats how i learned. i couldn't do lincoln's beforehand. i think it is more crucial to be able to backflip than lincoln