First you will need to get your tie dye and plain white shorts. You can also use light denim shorts (if the denim is dark, bleach it first. There are a lot of tutorials on how to bleach shorts). Set out a tarp or a large trash bag over your work area (Do NOT use paper towels or newspaper because the dye will soak through).

Soak your shorts in soda ash. Some tie dye kits come with it (your kit should also provide directions for using the soda ash). The purpose of using soda ash is to lock in the color. I didn't use the soda ash and my colors ended up fading from red to pink and from yellow to almost white :(

Place your shorts on your tarp after you have soaked them in the soda ash. I laid out a trash bag in my driveway to dye them. You can usually dye your shorts either damp or dry. Read the instructions on your tie dye kit. I dyed mine while they were dry.

To dye the shorts: just squeeze the dye out of the bottle in alternating colors. You can use whatever colors you like! Some of it will soak through to the back of your shorts but that's okay because you will be flipping them over and doing the same pattern on the back anyway. (TIP: DO NOT put red in the crotch area of the shorts or you might receive period jokes. And if you put yellow in the crotch area you might receive pee jokes. People can be so cruel xD)

Let the dye soak anywhere between 6 and 24 hours or whatever the directions on your tie dye kit say. The longer you leave it on, the longer the color will last. This doesn't mean leave it on for several days though :P

After the time has passed, rinse your shorts in the sink until the water runs clear. Then throw them in the washing machine on the highest heat safest for the fabric. Put them in the dryer and there you have it! Your rainbow shorts are ready to wear! :)