First you will need:
1) pair of tight trousers
2) material fo the extra part
3) sewing machine
/ hand sewing eqipment
Put on your trousers of choice and carefully mark where the knee part lies. If your trousers are very worn and have pale looking knee-marks, then there's no need to try them on just mark them as the knees.
Carefully slit the seams of the trousers till you reach the afore-mentioned mark, both seams for each trouser leg.
You will now have trousers with two slits in each pant leg. Taking the extra material, place in betwen every slit, so that the trouser pants can be widened to the desired proportions. The resulting extra material will look like a triangle, when vewed through its niche in the trouser leg.
Pin or hand sew the extra bits to the trousers. This can be trickier if the material is very thick especially if the needle of your sewing machine is not appropriate for sewing through certain material, like if you're using denim.
The border of each leg can be decorated with ribbon, or pre-bought embroidered borders one can find at any haberdashery.
ta-da, there you have it, instant cool hippie-wear and very easy to make. :)