How do you get a girl to let you finger?

If she doesnt want to be and says no then let her, don't force her into it. a time will come in a relationship when she feels free enough to.

if you know that shes okay with it, but isn't in the mood some tips would be to tickle and kiss her neck, stroke up and down her leg and kiss her pasionatly. this can turn most girls on then puuting her in the mood