I used to do this to my girls' jeans after school let out. Great way to boost the summer wardrobe for next to nothing! Transform an old pair of jeans into adorable summer shorts using bias tape. Instructions on the site.
Because t-shirt fabric is stretchy, it is really easy to make ruffles. You need to stretch the fabric out at the sides while pushing forward at the same time, This creates the perfect t-shirt fabric ruffles without having to mess around pulling threads.