Step 2: Making the shirt!
1 of your background
1 of you wearing your t-shirt
then import it into your foto editing program ( i suppose you have 1, otherwise download a trial of corel paint shop pro ).
click on your foto with you on it and go to layer->background layer->promote background layer
then go to your background and copy paste it on your own layer then move it down, you can now close the background window
in the layer window move the background under your own layer, if needed erase some background on your own layer so your real background is visible
then duplicate your own layer so you have 3 layers
go to your own layer and click on the little eye next to your copy then erase the t-shirt, if you dont do it very precisely you get the best look
put on the copy layer and put opacity to 50%