Cut a piece of dry film with the size of your board.
Strip off one of the cover sheets which are primitively adhered to the dry film.
Adhere the film to the PCB. (Be careful of bubbles!)
Place a clean sheet ( a piece of cloth ) on top of the PCB and iron all over the sheet with approximately 110 degree Celsius heat. ( Be careful for excessive heat which could melt the film).
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From : http://www.warapcb.com/dryfilm.php