Ray casting

I want to implement a volumetric visualisation using the technique of ray casting. I''ve researched quite a bit on volume visualisation and have found two main techniques: isosurfacing and direct volume rendering (dvr). I have implemented an isosurfacing technique: marchin cubes (using opengl); and now i want to implement a dvr technique, one of which is ray casting. I''ve found some material on what it is and very sketchy algorithmic details, but i still don''t exactly know where and how to start. Any help regarding how to start implementing will be great. thanks.