If you are on Windows 10/11 or Android and have downloaded a free mod from a site like MCPEDL, CurseForge, or a creator's Patreon, follow these steps.
Example command (rough): /fill ~-10 ~10 ~-10 ~10 ~10 ~10 water (then break the bottom blocks) tsunami mod minecraft bedrock install
This is the most common method for installing external mods found on community sites. If you are on Windows 10/11 or Android