You kinda don't... The diffuse map should really only carry diffuse information and light baked in if you're doing that too. You can't bake the roughness or metalness of an object in really.
Not sure if I'm missing something here but yes, and in most cases you don't need to do anything. Just plug your Diffuse albedo map to as you diffuse and you should be set.
Since the diffuse/albedo map is just a diffuse map with no light information sure...? Unless you mean, bake down all of the information? You could do this too You could also take a look at the baked lighting utility in Substance Designer/Painter which isn't PBR but will give you a nicer looking result than all of the PBR…
sorry i should of been more clear, i mean like baking everything down and what are the best maps to bake into the diffuse? im so used to working in pbr that i am finding it hard to do it
What you want to bake in depends greatly on what the target Shader does. Assuming we're looking at a phong with diffuse, spec and normal it'll help to multiply the ao map gently over the top of the albedo and add a gentle highlight to anything that pokes out - you can derive that from height if you have it or ao if you…