like in this video
The rainbow is probably done by folding the shirt in half and doing accordion folds... and maybe the clovers are done with dental floss, like the mickey head?
Here's a tutorial using a different folding pattern:
Ok, so here are my very first beginning efforts:
|the colors are all wrong, but what I did here was to hand draw the leaves of the clover and then bunch up each section and rubber band them off|
|here I decided to actually use the dye as paint and use a paint brush to create a shamrock, only problem is it runs|