We'll start by creating our buffers. We'll use a framebuffer object with a depth attachment and two color attachments. The ambient component can be stored in a renderbuffer (as opposed to a texture) because we'll blit (a fast copy) it over to the default framebuffer rather than reading from it as a texture. The diffuse + specular component will be stored in a texture.

Create the ambient buffer (ambBuf), a depth buffer (depthBuf), and a texture (diffSpecTex), then set up the FBO:

glGenFramebuffers(1, &colorDepthFBO); 
glBindFramebuffer(GL_FRAMEBUFFER, colorDepthFBO); 
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT,  
                          GL_RENDERBUFFER, depthBuf); 
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0,  
                          GL_RENDERBUFFER, ambBuf); 
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT1,  
                       GL_TEXTURE_2D, diffSpecTex, 0); 

Set up the draw buffers so that we can write to the color attachments:

GLenum drawBuffers[] = {GL_COLOR_ATTACHMENT0,  
                        GL_COLOR_ATTACHMENT1}; 
glDrawBuffers(2, drawBuffers);