1) Cut up your shorts to the length you would like them. You can just cut a straight line across, but I prefer to have them a bit longer at the back so my bum doesn't hang out! Keep trying them on to make sure they are the perfect length. Also allow for freying.
2) Put them in the washing up bowl and pour the whole bottle of bleach over them. It does take a while to work, so leave it for a day. I know it's annoying to wait :( Every now and then put on your rubber gloves and move them around. If you don't there will be dark blue patches.
3) When they are done, put them in a "rinse wash" in the washing machine to rinse out all the bleach. On mine the option is "Freshen up/rinse". Put them in separately, on their own.
4) Now fill up the washing up bowl with cold water, enough to cover the shorts. Pour in the sachet of powder dye and stir it. Put your shorts in and leave them for about an hour. Make sure to move them around (wearing gloves) every now and then so the colour is even.
5) When they are done, leave them to dry on the washing line or on an airer. They should now look something like this :) I think this dye is a little too light so next time i'll use a darker one. To frey the edge, get your scissors and scrape along quite roughly and the threads should start to come out. Then pull them down untill they are long. You might want to put them in the wash now, as this will frey them a lot more and make them look nicer. It might also fade the colour though, so it's up to you :)