After downloading a .tree file containing the tile information for a planetary surface layer, it can be installed by copying to folder
where <orbiter-root> denotes the Orbiter root folder (the folder containing the orbiter.exe executable), and <planet-name> denotes the name of the celestial body for which the high-resolution tileset is to be installed, for example.
Before copying the new tileset to its destination, you should back up the existing original low-resolution version of that file. This allows to go back to the old version in case of problems with the high-res textures. Simply rename the existing .tree file to .tree.lores.
You do not need to install all high-res layers for a planet at once. For example, you can install the high-res surface textures (Surf.tree), but retain the low-res elevation files (Elev.tree).
Installing the MinorBodies package: The MinorBodies zip file contains surface layers for several celestial bodies. Extract it in a temporary directory and copy the files to their target folders as described above.
A: The torrent download is recommended, because it is more robust and doesn't rely on the download server bandwidth. Only use the Http download if you can't use torrents.
A: The differences only become apparent close to the surface. At high altitudes Orbiter uses lower resolution textures, so it will look the same.
Note that the default low-resolution packs do contain some local high-resolution areas as "showcases" (for example Kennedy Space Center and the Alps on Earth, or Valles Marineris and Olympus Mons on Mars), so these will not improve any further with the high-res pack.
A: Some of the texture files are very large, so you need a reliable high-speed internet connection to download them. You may be able to use a download manager that allows continuing a failed download. The torrent download is generally more reliable.
Also make sure you are downloading to an NTFS file system, not FAT. FAT file systems (commonly found e.g. on flash drives) don't support large file sizes (see below).
A: Open "This Computer" in the Windows file explorer, right-click on the drive you want to install Orbiter on, and select "Properties". The file system type should be listed under the "General" tab. Make sure it says "NTFS". FAT file systems are not suitable for downloading and installing the high-resolution texture packs. FAT file systems are mostly found on older hard drives and flash drives.
You can check the integrity of the downloaded file by computing its MD5 checksum and comparing with the value listed on the download page. You may need to download a Microsoft or other 3rd party utility to compute the MD5sum.