Fast Smelt is a data pack, so it is enabled per world rather than as a game-wide mod. Once it is active, every furnace, blast furnace, smoker and campfire in that world cooks each item in a single game tick.
Adding it to a new or existing world
1. New world: On the Create New World screen, open the Data Packs option. Drag the pack file onto that window, or use the button that opens the pack folder and place the file there. Move the pack into the Selected column, then create the world.
2. Existing world: Close the world. Open your Minecraft folder, go to saves, then your world's folder, then datapacks. Put the pack file inside. Load the world again.
3. If the world was already open when you added the file, run /reload to load new data packs. You can then check /datapack list to see whether the pack is enabled.
About the "incompatible" message
The game may warn you that the pack is incompatible or was made for a different version. You can ignore this and continue. The pack still works normally. Confirm the warning and keep loading the world.
Quick check that it works
Place any furnace, add fuel, and put in one raw item. It should cook almost instantly, with no slow progress arrow. If it smelts at the normal pace, run /datapack list and make sure the pack appears among the enabled packs.