This tut will concentrate on using a pattern you may have found on the internet and thought, "now that would look great on a shirt".
First open up you pre-made template (if you've attempted Xaoz's basic shirt tut then that's an excellent one to add a pattern to). Here i'm using a bustier template i made a while ago.
Next open up the pattern you found and add it to your template as a new layer
Now at this stage (depending on the pattern) you could just stretch the image so it covers the entire template area.
I've decided that the image will be too big once made into the top, so i'll make it smaller then duplicate it so it tiles.
Make enough tiles of the pattern to cover half of your shirt template.Hide the underlying layers and merge the tiled pattern.
Duplicate this newly merged pattern, then flip it over and drag it so it covers the rear part of the shirt.
Again hide the underlying layers and merge the two.
Now hide the new merged pattern layer and the mesh layer and select all the area around your shirt template.
With this area selected, make your patterned layer visible and select it
If you look closely you will see the selection lines still visible against the pattern layer.
Then hit delete.
This will delete everything on the pattern layer that's outside of the shirt template shape.
Then hit delete.
This will delete everything on the pattern layer that's outside of the shirt template shape.