Ability Casting help?

Hello everyone! So I have started watching League Videos on YouTube. I have been noticing something and i was wondering how you do it. Lets use as an example. If I'm sitting in a bush, i want to see how far my Q can go, but I have it on auto cast. I heard that you can make it so if you press and hold it you can do just that, but I haven't found out how. Now if there is a way to do this, do you feel that it affects your casting? is there a delay? Also can you cancel it after you long press it, or are you kinda screwed? Well thank you for your help and Good Luck in Ranked =D