Is it necessary to apply to anywhere other than to add a file as LICENSE.txt for open source or free software licensing?

I am going to continue a game engine project I didn't engage in for a long time.I amatourly writed some core libraries using OpenGL with C++. Because I am not that into programming in general, I am asking this silly question about software licensing. If I want to license my program ,for example, with zlib, is it enough just to include the formal manifesto(full text of the license) in source code itself or add it as a seperate text file on root folder and put on github? If no, should I additionally apply to any formal foundation etc.?

Edit : I have tried to make the question as clear as possible. If the question is still not clear or any grammatical mistake exists, warn me.