You have to invest some time in the story mode to get the blueprints to be able to craft a molotov. If you're looking for a quicker unlock: start the survival mode (Basement), you will be equipped with the ability to craft that cocktail right from the start. Now you need to find all ingredients to craft it. In your starting room you find numerous ingredients, be sure to pick up everything you find before opening the door to the actual mission area. Some few ingredients to build your molotov might be missing, but you can find them close to the starting area. When you have build it by pressing down
, selecting the molotov in the lower left of the wheel, pressing
brings you to the crafting menu. Select all needed ingredients and finish the crafting process with
. Now wait for the Alien to appear again (which won't take too long in survival mode) and throw it at it. If you have managed to damage it with the cocktails blast it will scream and back off. But when it is already grasping you it is too late. So be sure to throw the cocktail when the beast is still a few steps away from you.