1. Create a new Web document
2. File > Place a high resolution image of an animal of your choice from google
3. Set your fill to none and stroke to white and .25 weight. Start creating triangles with your pen tool that connect at the edges. Make your triangle sides overlap so you are not adjusting so much at the end. You choose the size. You can choose to fill as you go or just keep a stroke color and fill later on.
Tip: try and create the triangles around areas of color. The smaller the triangles, the more detailed the picture.