Ported box is the same as building a sealed only it requires obivously the port. Thats where it can get tricky, not in the actual building but in the calculations prior. You have to account for port displacement as well as calculate the port length. I use WinISD for that, http://www.linearteam.dk/default.aspx?pageid=winisdpro
That is very useful for getting the proper port length. Dont worry if your sub isn't listed, just choose any and enter the size box you want, tuning frq etc...
Like the other guys said, JL's site is very useful so check that first.
My fav tutoria isnt working right now but here's the link in case it works later.http://www.sounddomain.com/id/sgrannat