fin attachment

Hey sorry if someone has already asked about this but i couldnt find any topics that would help so i'm writing my own...
anyway...i just recently got my first board...6'8 nsp surf betty and i'm trying to attach my fins now but i have no idea how...i've never done it before so i need help...its probably stupid to say its like they don't fit because i suppose they have to be really tight...but if anyone knows how to do it then please help!!
It has 3 fins and what i think is 3 fin boxes...again i sound stupid but i don't know what it's called
HELP!!
Thanks!! xXx
anyway...i just recently got my first board...6'8 nsp surf betty and i'm trying to attach my fins now but i have no idea how...i've never done it before so i need help...its probably stupid to say its like they don't fit because i suppose they have to be really tight...but if anyone knows how to do it then please help!!
It has 3 fins and what i think is 3 fin boxes...again i sound stupid but i don't know what it's called
HELP!!

Thanks!! xXx