Mods can be gotten either from the Nexus Mods website or the Steam workshop. In the later case, this can be done using a website or the guide below (Note that the guide mentions the App ID changes after updates, but it does not. While the steamdb link may be useful, just for checking the App ID, the Steam store page or Workshop page URL would be easier).
https://www.reddit.com/r/DivinityOriginalSin/comments/73y9g4/how_to_download_steam_workshop_mods_for_the_gog/

There should be instructions in the download, or on the mod page either on Nexus or Steam, for the install, especially if there any special requirements or dependencies. In general, mods distributed as pak files get copied to the '..\Documents\Larian Studios\Divinity Original Sin 2 Definitive Edition\Mods' folder, while files/folders are copied (or extracted) into the game's install folder (usually '..\Divinity Original Sin 2\DefEd\Data' or one of the subfolders in the Data folder).