rub yourself, finger yourself, grope yourself with the other free hand, and make plenty of moans while looking at him with a seductive look on your face. If all goes well, he'll start to do it for you!
as to get it started, tell him you have a little present/show for him
Honestly, do it whenever, girls masturbating is such a turn on, and I bet it would be the perfect way to get him to want to have sex. Like if you are on the couch in the living room watching tv, pull down your pants and go to town on yourself, finger yourself 24/7, and if it does not work all the time, at least you will be having fun
First open your pu**y and finger f**k yourself to get your fingers juicy. Now you can play with your clit to get yourself turned on real good and your clit sticking out hard and excited. Now if you want you can use both hands, one to finger f**k yourself and the other to play with your clit to make yourself cum.