What we need:
white/ light object
To get shorts you should simply cut off your trousers' legs. So while wearing them mark the desired length and use your sharp (!!!) scissors to cut it. Not that hard, on youtube you should find heaps of tutorials for this easy process.
Now comes the process off dying. For this you should follow the instructions on your textile color, normally there is a part explaining how to do it in a bucket. Luckily my clothing hanger was just big enough to stay outside the bucket, otherwise I would have had to build some kind of construction to keep it from falling into my bucket. In addition to that I also taped my trousers to the hanger, safe is safe.
Since I did not want textile color everywhere in my house I decided on letting it dry outside...
The result... I'm not that happy with it, because the coloring is not as even as I wanted it to be, I probably should have move to more often while the garment was hanging in the color... You learn from mistakes.