i use xampp, and make a new custom theme running on xampp so you just go to 127.0.0.1 to see your site
when you are happy then save theme and upload to real site
xampp is amazing for offline testing and developing.
Easiest way is to use a theme boilerplate (i write mine custom usually, however thats harder)
bones ( http://themble.com/bones/
) is the best IMO but it uses LESS/SASS instead of CSS. You can use CSS but its awkward because it was really written for LESS/SASS
good tut: http://wp.tutsplus.c...etting-started/