Create a new 'Brush' and manually adjust the settings like you see here, you could also use the Marker, Watercolor, Airbrush. Run start-sai.exe to install the textures. Extract the RAR into your Paint Tool SAI folder (install 7zip if you don't have winRAR). Here are a couple of pencil brushes that I made for Paint Tool SAI in order to mimic graphite pencil, to install them follow these instructions*:
