I love to be grabbed on the outside of my pants or underwear. I don't know why it turns me on so much. You will learn by experience and paying attention to the guys reactions. There is no set way because everyone is different.
For your second question, ask for feedback. If you do something wrong, chances are he'll stop you or guide you in some way. My guy likes it when I flick my tongue quickly back and forth right underneath his tip, or drag it in circles around the tip while squeezing my lips lightly around him. Idk, it's easier done than explained, haha. But seriously, just ask him if he likes it. Or do like I do and before you start say, "The only sounds I want to hear coming out of your mouth right now are appreciative noises, suggestions, and occasionally my name." ;]] It makes asking him if he wants me to do something more or different less awkward.