For one of my final projects last semester I decided to learn how to create polygonal portraits in photoshop. I scoured the internet for tutorials and finally found one that is incredibly well done. They break down each step and explain everything. Click HERE to watch the tutorial.
The first time I attempted this is took several hours. There are definitely small hints you learn for yourself along the way. You'll find it pays to be very careful about following the grid and making sure there are no gaps between your polygons. By the time I was making my last 2 they only took me 30 minutes or so each. Like many things, it gets easier and better the more you do it. I even started experimenting by not limiting myself to only triangles and making other shapes. I'm happy with how they turned out and was surprised to find it relatively easy once you get there hang of it. They were quite fun.