blog post or the Sharpie website for detailed instructions on how to Sharpie tie-dye.
The Sharpie website says, "After your shirt is completely dry (alcohol is flammable, so don’t use heat on a damp shirt), set the colors by tumbling the shirt in a hot clothes dryer for 15 minutes." I tried this and the flowers still bled a little in the wash. So I'd suggest washing your shirt alone until you are certain the ink is indeed set.
Let me know if you have any questions.