It's not that complicated. Just take a container width of 960px or 1280px divide it between the amount of columns you wish to have, the result is the column width (without inner or outer padding). You can use %, em's or px's but the same principle applies. It all depends on the nature of your project and what device the end users favors.
As a seasoned front-end dev my advice is not to use html based grids (like bootstrap). In the long run, you'll end up spending time adjusting the HTML file AND the css and you'll have not learned anything. Also, your project will end up looking horrible and you'll have to hack it all with :after's and position:absolute and whatnot.
Bootstrap/foundation is wonderful for concepts and temporary gigs. But really, if you really want to learn? Make your own grid on codepen or something based on what I said above.