Most questions, like yours, have likely already been asked many times. If you use the forum search bar on the main forum page, or the site search in the top-right of any page on the website with the text, "Open Chakras", then you'll get quite a bit of pages to read on the subject. Just as you would if you Googled it.
I say the above to help you in the future.
To answer your question, you need to do nothing to open them. All of your chakras are open. Chakras are energy centers/pools of biomagnetic/electromagnetic energy in the body. If they were closed, you'd be dead. We need energy to live.
What most people have are blockages, which restrict the flow and growth of a specific or multiple chakra centers. This could be from a variety of reasons depending on the chakra.
You can work with and develop your chakras through yoga, such as "Hatha" or "Kundalini" yoga, through vibrations using seed sounds - such as OM or AUM, through mudras, and through dealing with emotional baggage. There are other ways, but you get the idea.
I suggest taking what I've said and Googling for more information. Read a variety of sources, pick out the parts you're comfortable with, and experiment.