Love, Love, Love..
Have it up on mind. Always.
You like your partner. You like his/her closeness. Partner's face. Body, shape.. Legs.. and everything you see with [or without] clothes. Think of them whenever you have seen those. Try and think of some pleasure your derive out of the sight or touch of it. Sometimes you would have gone to some organs also... ooh are they again on your mind. That is it... that is the start of True Love.
Imageine, he/she is around. Close, next to you sitting. You can smell her/him. You want to touch her/him.. a desire will be born.
Nurture this desire. Let it increase..... have it whenever you are alone... in bath, in bed, in class room, in office.. wherever. Practice thinking a love mixed with desire.. you will only add to it. It is treasure that would always increase. So why shy away.. it is your mind and you.. no body is looking or listening.... feel free.. that is the first principle....