I usually shrink things by default. Not setting the washing machine correctly or something like that. Anyway if you want to shrink your jeans have a go at the following.
Try boiling them first. Pop them in the washing machine and set the machine to the highest heat setting. They can be either tumble dried or on the line. If they are new and havenít been preshrunk they may shrink by up to 15%. Using Alum (see below) will give you the same result unfortunately neither are totally permanent as fibers stretch to a certain degree with wear.
Aluminum sulphate also known as Aluminum Sulfate, Alum; Kalinite; common alum; white alum