To install Extrasolar, follow the steps below:
1) Install Kopernicus
2) Move the "Extrasolar" and "Extrasolar_Textures" files from this zip file into KSP's GameData folder
3) Done

To Install visuals for Extrasolar, follow the steps below:
1) Install scatterer
2) Install Environmental Visual Enhancements
3) Install Stock Visual Enhancements (if you are using After Kerbin with Extrasolar, skip this step)
4) Move the "EXVE" file from this zip file into KSP's GameData folder
5) Done