I've NO idea how much tie-dye experience you have... but just in case the answer is... "Not much" here's a little tutorial-ish thing telling you all about how I do it!
You will need:
1 pkg of RIT dye
An old pot and wooden spoon
tarp/plastic drop/some kind of covering for your work surface
Step 1: mix dye according to instructions on the package... You obviously make a stronger solution for more vibrant colors, and weaker for pastels. Let it heat on the stove while you prepare your work area and onsies. Spread plastic drop over your work area and be sure to have rubber/plastic gloves handy when you start handling the dye.
Step 2: Wet your onsies with water... and ring them out well. You get a nicer design if you go ahead and button the crotch of the onesies before you twist and band.
Step 3: Twist and band. There are hundreds of different designs and ways to tie-dye. I am just going to show you my two favorite... they probably have official names, but I call them swirl and rocket.
Mine pictured are all pink (lots of friends having baby girls...) but I have made blue ones in the past that are just charming for a little boy.
Happy Tie-dying! :)