First you gotta login (which can be tricky for some, they send you a code and stuff)
Then You can chose from the color selector at the bottom and place your pixels onto the canvas. The template just makes it easier to view where one should place their pixels to complete the picture.
For the color selection, it's a lot easier on pc. There you can just middle-click to automatically select the color of the template. On mobile you have to find that color yourself in the color selector at the bottom. The body color is purple as an example (there are multiple shades of pink).
The template is just an overlay over what has already been done. In the "Dotted Big" mode (which is activated in the link I put into the post) the templated pixels are the small ones and the larger pixels are the ones which have already been placed by others. So essentially, we want the pixels to go from small (only in the template) to big (actually placed on the canvas).