I have a few tips about tye dying.
Make sure you squeeze out the shirt really well after soaking in the soda ash. It prevents the color from bleeding as much.
When you put the color on, put colors next to each other that will mix together well, in case they do bleed together. Ex. blue and yellow next to each other will make green, but purple and yellow will make brown (not too pretty, had to trash that shirt).
Start with lighter colors first then go to the darker colors.
I do the Mickey part last and wrap it in plastic separately so no other color transfers onto it while it's setting. (Unless of course you want it to)
One last thing, you don't have to use a traditional t-shirt. I used girls undershirts that come 3 in a package, much cooler for hot weather not to mention cheaper. I also used camisol tops from Old Navy but not until I had alot of practice on the chaep shirts first.
Have fun, and don't worry, they're tye dye , they're not supposed to look perfect.