It is easy to tie-dye denim once you have removed the initial denim colour, unless you have white denim.
I simply zig zag (like pleating) folded up the length of the legs separately. I then used elastic bands to hold my pleated fabric together. I used several elastic bands along the width of the folded fabric.
I them put the jeans in a bucket and poured house hold bleach over them. I like to leave denim in bleach for 45 to 60 mins. Then remove the jeans and place in a washing machine on 40 degree wash - use gloves!
Then all you have to do is removed elastic band s and pop back into the washing machine with a washing machine dye! Again I use a 40 degree wash.