@Amethyst is right in that you should follow a playlist and do all of them.
Do them in order, that means start with the lower chakras and work your way up so that when you get to the third eye chakra you have an easier time staying grounded.
I recommend this playlist on Youtube. Each chakra has an activating mantra that you have to repeat.
Keep in mind, that is for activating the first chakra, i.e. cleansing it.
When people mention “opening the third eye” they can mean a variety of things. As far as I understand, the third eye won’t suddenly open like in The Matrix. 
It’s a continuous process of achieving knowledge and enlightenment through hours and hours of meditating and deep, mind-altering realizations.