Cast on method explained (like my favorite): Exerpt - Personally, I tend to cast-on rather tightly, buy more yarn than I end up needing for projects, and I'm never a fan of having to go back and redo my cast-on, so I allow for about a 2.5cm (one inch) tail for every stitch I'm casting-on (if my yarn is bulkier I may do more than that). This always leaves me with more than enough yarn, and you could easily do less than this if you wanted to.
When adding a new color, never knot! Instead, join new yarns neatly and invisibly at the beginning of a row. Knots have a way of coming untied and working themselves to the front of your work when you least expect it. We recommend this method: Place the needle into the stitch, dropping the old yarn and picking up the new yarn, leaving a 4-inch tail to be woven in.