I used to have purple hair a few times before and loved it, depending on the brand and how well your hair accepts it it should last 4-6 weeks. With any of the semi-permanent hair dyes they fade little by little every time you wash your hair , just make sure you wash you hair with cold water it will allow for the color to stay longer.
Erm.. all you have to do is.. If your hair isn't blonde, then bleach it ( you don't have to but the color won't show up well on other hair colors ) So bleach your hair, then apply the dye onto clean, dry hair, leave it in for like a hour, then wash it out with cool water ( don't use shampoo it washes out the color )