Practice practice practice. I do not remember learning, but I suspect I got the hang of it while on my first bike tour when I was 15. That was a long time ago.

Start by just touching the waterbottle while on your bike. Do it by feel until you feel comfortable just with reaching down and learning where the bottle us by feel.I never look at my water bottle; my hands just know where it is.

I carry two water bottles-one on the seat tube and one on the down tube. I can use either one, although I find myself reaching for the one on the down tube more often.

Be patient. And if you drop it (I have), don't panic. Just slow down and go pick it up and try again.