Next stage is to add the solder mask. All handling of the mask, up to developing, should be done in subdued light and out of direct daylight or any UV source, as it is light sensitive. You first apply the mask using a laminating machine, I modified mine slightly as the opening was not very tall, plus its easier to see what's going on, I just cut away some of the casing. I peeled off the protective backing on the Green film and laid it on the PCB, I ran it through 4 or 5 times. I then cooled it rapidly under the cold water tap, this was not in the instructions but a couple of tutorials on YouTube recommended this to aid adhesion. I found it best to tape the leading edges of board/film together and hold the film up as it was fed into the machine, this seems to avoid creases and air bubbles.