It's pretty much exactly the same thing except you multiply everything by a matrix that looks something like this instead of the standard projection matrix:

    2/screen_width,  0,               0, 0.5,
    0,              -2/screen_height, 0, 0.5,
    0,               0,               1, 0,
    0,               0,               0, 1

Then assuming that you are drawing back-to-front and don't want to do depth-rejection you can just disable depth testing:

    glDisable(GL_DEPTH_TEST);