One of the things we have a lot of and need a lot of is PJs. The kids don't like the shirt style and prefer to sleep in a tshirt with either shorts or pants dependant on the season. I always decorate the tshirt to match otherwise it just ends up in the tshirt drawer never to be seen again. One of the quickest and easiest ways I find is with machine applique.
tshirt - I just buy commercial ones from Kmart or Target for under $56
Fabric for applique - in this case a panel from Dr Seuss Lorax cotton
Vlisofix - from Spotlight etc usually behind the counter. It is an iron on double sided sticky film.
Also you will need an iron
1. Cut the vlisofix to side. Make sure you keep the film connected to the paper otherwise you will end up with a big mess on your iron.
4. Position your fabric where you want it on your tshirt. Then secure the image with the iron. The heat bonds the glue so the image and tshirt are now stuck together.