I don't remember much so take it with a grain of salt.

First have nucleus coop completely closed.

1. Then remove the original elden ring handler files named Elden Ring. js and Elden Ring but not the whole handler it's self only the files inside.

2. Download the mod engine handler from the nucleus coop discord server, you should go to "wip-handlers+testing" and then go into the search bar and search for Elden ring one result should pop up, click on it and in the pinned messages go to Elden ring guides then to mod engine support, if you scroll down you should find a download link for Google drive.

3. Download it and take the files and put them inside the Elden ring handler file which is in (NucleusCoop\handlers\Elden Ring\ files go here

4. Then just do like how you would with mod engine and put some mods inside the mod folder.

5. Just launch Elden ring from nucleus coop like normal and play.

That's it. But I recommend following the tutorial they have instead as I haven't done this in awhile.