It is a little difficult. I found a tutorial on it the other day which shows it is nowhere near as difficult as it looks though. If you find a picture, some art or a drawing you want to make into some Poly Art, you then draw around it very loosely, draw around some main details loosely and then fill everything else in with random lines that connect to make triangles and when the entire image is filled with different random triangles, you make it so the lines create the shapes and then you select the colours from in image underneath so the triangles can have some accurate colour. This is just how I did it in Illustrator though so it's probably very different in another piece of software.
Here's the tutorial if you are interested.