Generally, you open the Makefile with Programmer's Notepad, then click Tools and make. That's all you have to do...
But, if it's one of the 1st homebrew, you'll have to re-write some parts of the code because libogc has changed since then. It could be trivial, rename a few variables/functions or a real pain with large portions of code...
Sometimes, you'll also need to install extra libraries(sdl, freetype,etc.)...