There is no need to properly "install" ORCA. All you have to do is go to the ORCA forum website, click on the upper-right button to register and enter. Then click on the "Downloads" button
and choose your ORCA version and operating system (we recommend the newest ORCA 5.0). If you have doubts on which file to pick, take the first one for your OS. On the same page, you can find the recent manuals and some other important files.
After downloading, all you have to do is extract the files (or run the installer) and you are ready to go.
Don't forget to add ORCA do the PATH variable (what is that? Assigning PATH variables ) to call it from anywhere, or add the full/path/to/orca from another folder.