Choose your brush tool. Right click on the document, and look for an option that says "Load Brush Preset." Find the file you downloaded and look for that. Then the brushes should be added to the end of your list of brush shapes.
In game you only see the lines on his face, his upper arms, feet, and the palms of his hands. (And I wasn't able to match those perfectly.) So the rest you just really have to guess on. I went with just tracing muscle groups, with a bit of the Dalish tattoos mixed in.