The instructions for installing instant client are at the bottom of the download page:
Installation Steps:
- Download the appropriate Instant Client packages for your platform. All installations REQUIRE the Basic package.
- Unzip the packages into a single directory such as "instantclient".
- Set the library loading path in your environment to the directory in Step 2 ("instantclient"). On many UNIX platforms, LD_LIBRARY_PATH is the appropriate environment variable. On Windows, PATH should be used.
- Start your application and enjoy.
You have only downloaded the SQL*Plus package, which is described as "Additional libraries and executable ...". The word additional is important.
You have to download at least the basic package (or basic-lite which has less internationalisation support) to be able to use any instant client features.
All of the packages you download have to be unzipped into the same directory. You can then put that combined directory anywhere you want, and point your PATH environment variable to it.
There is more information about Instant Client on Oracle's website. That is generally a better place to start than a third-party blog.