Open the SKSE archive. Copy every file inside the folder (especially skse64_1_6_640.dll , skse64_loader.exe , and skse64_steam_loader.dll ) and paste them directly into your Skyrim Special Edition folder.
This is usually found at: C:\Program Files (x86)\Steam\steamapps\common\Skyrim Special Edition (Tip: You can right-click Skyrim in Steam > Manage > Browse local files). skse 16640 link
Many modders intentionally "backport" or freeze their game at 1.6.640. This is because the transition from 1.6.640 to 1.6.1130 involved significant changes to the way the game handles plugins (the "Marketplace" update). Open the SKSE archive
Inside the SKSE archive, there is a Data folder containing scripts. Copy the Scripts folder from the archive and merge it with the Data folder in your Skyrim directory. Many modders intentionally "backport" or freeze their game
By staying on 1.6.640 with the corresponding SKSE link, you ensure:
For game version 1.6.640 , you specifically need SKSE64 version 2.2.3 . Using a newer or older version of SKSE will cause the game to crash on startup. How to Install SKSE 1.6.640 (Step-by-Step)