I'm not the best at these kinds of things and I doubt I ever will be, but it depends on where you are and how often you see them. If it is similar to school or work then just warm up to them and have...
Go up to them and say hi. Compliment them and walk away. Next time they see you they will realise they like you. Its like puting in a good word for yourself.
JUST GO UP TO HER AND SAY HI AND YOUR NAME.AND DONT MAKE IT SEEM LIKE YOU LIKE HER,JUST COME ACROSS AS FRIENDLY(BUT NOT TOO FRIENDLY).THEN AS YOUR FRIENDSHIP PROGRESSES ESTABLISH THE FACT HTAT YOU...