I am posting this link for hemming jeans, found it on another sewing forum, the language is German but the pictures should be self-explanatory. Hope you find it helpful!
After posting and testing the link I think it is necessary to add that after you cut the original hem off and shorten the jeans to the required length, then you re-attach the original hem. Somewhere in the post is the maximum length which can be removed from the jeans legs, otherwise you will find that the width of the legs changes and the hem might no longer fit.
Edited 11/8/2005 3:10 pm ET by Desiderata