Thank you to Char at Crap I've made for her advice and tips on applique and for her Jolly Roger template.
I bought a black vest and chose a bright pink fabric for the applique. I added a little heart in her back to make it cuter.
Applique is really very easy.
I used HeatnBond iron-on adhesive. I printed the pattern on a normal sheet of paper and placed the adhesive sheet on top of it. The adhesive is transparent enought to see through so I was able to trace the pattern directly onto the adhesive sheet.
Bond with the applique fabric. Cut around your pattern. Iron-on the T-shirt. DONE !!!
I used the no sew HeatnBond as I knew the T-shirt was not going to be worn over and over again. I suppose it is longer lasting if you sew it, but I washed it and it is still bonded...
My advice: really read and follow the instructions for the adhesive. When I first tried it I hold the iron for too long on the applique fabric and it never bonded to the T-shirt....
And don't forget: if you are going to applique a letter or a number, you need to reverse your pattern.