This was my first double exposure I have ever taken on my film camera. What it involves it taking one exposure (in this case for the colour in the sky) and then taking another exposure (in this case for the lights and fireworks) without winding the film on. For this image 20 or 30 minutes elapsed between exposures. The reason I did it for this particular frame is that when the fireworks were due to start, the sky would have had no colour.
This can be performed digitally using Photoshop and I have done this previously (and also wrote a blog about it --> http://www.martincanning.com/blog/2013/9/exposure-blend---santos-glng-city-of-lights) but I feel that doing it on a piece of slide film with no photoshop is much more rewarding!