How do you convert a cake recipe to make brownies?
Brownies are typically dense and less fluffy compared to cake.  What you want to do is to make the cake recipe more dense to make it more like brownies. Here are a few factors that would effect density/fluffyness.


Number of eggs: Adding more eggs will make the cake more dense
Preparation of eggs:  Beating the eggs will cause the cake to be more fluffy since you are introducing more air into the mixture. You should stir them lightly with a fork (not beat/whisk them) until just liquefied, to avoid a fluffy cakelike texture. 
Addition of baking powder.  Most cake recipes call for baking powder while many brownie recipes do not.


I suggest reviewing some brownie recipes and the comments to get a feel for a good starting point for the cake recipe modifications.