If you have disabled automatic updates on Steam and your game version is currently 1.6.342, you use SKSE version 2.1.2. Using a newer or older version of SKSE will cause the game to crash on startup or display the "Version Mismatch" error. How to Install SKSE for 1.6.342
Copy the contents of the Data/Scripts folder from the SKSE download into your game's Data/Scripts folder. skse 16342
Go to the archived SKSE builds or the Nexus Mods page. You are looking for skse64_2_01_02.7z . If you have disabled automatic updates on Steam
For a step-by-step walkthrough on getting the script extender set up and verified: skse 16342