Using a snapshot in order to later fill it with watercolour is extremely easy. First, you need to make sure that you have the picture that you want to sketch and paint.
Grab a lamp and a transparent table that will allow you to put the lamp right underneath the glass on which you will put the snapshot and paper. Once you have done so, put the snapshot right on top of the glass top with a lamp underneath. Then put a light paper on top of that snapshot and you will observe how each and everything is visible to you.
Take a pencil and start sketching. Once the outline has been made, turn the lamp off, put the snapshot away and start painting. You can even use the lamp in order to grasp the idea about the colours which have been used in the picture. This will help you copy the picture in the best way possible.
Using slides is also a brilliant idea in order to complete a sketch that you later want to fill through watercolours. You will need a projector that allows you to make the focus screen large and even small, according to the size of the paper that you want the picture to be in.
Make a slideshow on your computer and position the projector in such a way that you can easily sketch everything that it is displaying on your page. Once the sketch has been made, turn the projector off and start painting with the help of watercolours.
The projector is difficult to handle when it comes to painting as it keeps casting your own shadow on the page that you are trying to sketch on.