Grab Nif Optimizer and follow its instructions to convert your files.

Unpack your files to a temporary location somewhere. This step should be done before trying to load your mod into the CK as well since the same problems will occur with the CK - it'll either hang or crash. The game will either hang or crash at the opening menu. If your mod uses a BSA file (and pretty much any decent one ought to be if it has assets to provide), you can't simply use that file with SSE. If you have meshes, you'll need a copy of the SSE Nif Optimizer program. You'll need to have access to the current SSE CK and a copy of 圎dit 4.0.1 or higher at minimum. It also assumes a certain level of familiarity with the packaging of files relating to the mod. This guide is intended for mod authors to use as this process is not suited for end users to follow due to the details that need to be addressed to do the work properly. The following rough outline of the process should be enough to guide you through porting your mod over to SSE. They must be properly converted over so that they will function correctly in SSE and not lead to save corruption or CTDs. Due to differences between LE and SE, they can't simply be dropped in. Many fine mods have been created for the Skyrim Legendary Edition (aka Classic) over the years and not all of them have been brought over to the Special Edition.

Skyrim mods manually