What you will need;
onesies
iron
ironing-board
iron-on appliques
cloth
Place the apllique where you want it. Put the cloth over it. Make sure the apllique doesn't move. Heat your iron. I use the wool position.
Press about one minute, let it cool 1 minute, press another minute. Instructions may be different, so read them before you begin.
And this is how it looks like. Very easy, simple and quick to make.