Well its just as hard as any other model, the process of modeling is the same, but it does take a lot of time.
If you get a lot of good reference images, modeling sheets, and watch a few tutorial, you should be good to go. Keep in mind that modeling a human or any mesh that will deform you have to be careful about poly loops or poly flow.
As Lostsciene mentioned, MakeHuman his a free human.obj generating program, you can bring them objects into blender and reduce their polycount for the game engine.
if you want to it from scratch, the process is the same as most models, start with a cube and a mirror modifier and match vertices to reference photos. the photos are going to be your saviour when it comes to people. 3d.sk is a paid site but has lots of free reference photos