No idea what a symlink is? https://www.howtogeek.com/howto/16226/complete-guide-to-symbolic-links-symlinks-on-windows-or-linux/
I recommend using the linkshellextension tool if not familiar with the commandline, very simple to use.
Install the warframe launcher (only a few MB) and note where you installed it (ie C:ProgramFiles(x86)EpicGames) on epic and claim the skin pack from the store.
Locate your warframe install by rightclicking the launcher -> open file location. If you end up in the start menu folder, do it again to find the actual game install with the .exe files, then go up a level. For a steam install, rightclick -> properties -> local files.
Then just symlink (with LSS right-click “Public” -> “pick link source”) the folder (standalone warframe install location)WarframeDownloadedPublic” folder into (epic warframe install location)Warframe, and rename the link to “Downloaded”.
Public/Downloaded contains all the game files that would otherwise be downloaded when you run the launcher.
Open Downloaded and rename Warframe.x64.exe to Warframe.x64.exe_standalone, so epic will use the epic-specific version. Switch it back afterwards.
EGSWarframe folder should now contain: .egstore, Downloaded, Warframe.exe, Warframe.zip.
Then run the game via EGS and the launcher will locate your existing install through the folder symlink.
The skins are for Stahlta, Ogris, and Drakgoon, have new sfx/vfx, and come with the basic version of each weapon and a catalyst.