You need a mesh and a texture of your new weapon, then you have to add it to your new module. When you are done, the rest boils down to modding your newly-made Molotov Cocktail. Here are some OSP kits to help with your goal.
If you have a bottle model and trying to add some flame effect, I think it's done by adding some particle system to the item via item triggers.
Perisno mod has a flaming sword called Flamebringer. You may ask them. It's a warband mod but these games share the same set of operations cause of the engine.
You need a mesh and a texture of your new weapon, then you have to add it to your new module. When you are done, the rest boils down to modding your newly-made Molotov Cocktail. Here are some OSP kits to help with your goal.