It might be a good idea to practice a couple times on a piece of scrap paper to get the brushing technique right.
Cut a piece of freezer paper the size of the front of your tee and iron it onto the inside of your tee.
Now turn the shirt to the right side (the freezer paper should be between the 2 shirt layers. Add some paint to a piece of scrap paper
Brush the paint onto the shrit gently. The freezer paper should prevent too much bunching but the less stretch in your shirt the better!