How to build a ported subwoofer box?

I have a Herts He2 amp and an alpine 6013 strontium sub woofer (Which is currently enclosed in a sealed box with.7 cu ft) in my honda civic hatchback. I was thinking of building a ported box for the sub but i don't know a whole lot about how much extra cu ft is needed or how to tune the port properly hence my asking online… So any responses are much apreciated.

Subs generally come with a manual which contains dimensions and instructions for building both sealed and ported enclosures that compliment the subs capabilities. Look into that. Just building a ported box or going out and buying one without an idea of the airspace and port size that is compatible with your sub will be a waste of money and produce less-than-best sound quality.

Judging from 6013's specs (fs:30Hz, Vas:96lt, Qts:0.31) and using an online box calculator you get the following results:
You'll need a ~1.4 cu ft box -internal volume- with a 3"x7.5" port (Diam x Length).
You shouldn't expect louder bass but you should expect deeper bass as 0.7 cu ft sealed box goes down to 68Hz at -3dB while 1.4 cu ft ported box goes down to 45Hz!
In case you need to experiment more on box or port check below calculator. Good luck.
http://www.mmsa.org.uk/guides/car_audio/subwoofer_box_design_calculator.html