For my mod, i wanna add a second armorlayer below the normal one. How can i make that? What are the things i need?

I wanna render some clothes above the players skin

Okay.

Is that the right way of doing it?

 

public void onRenderPlayer(RenderPlayerEvent e){
	e.renderer.addLayer(new LayerRenderer() {

		@Override
		public boolean shouldCombineTextures() {
			return true;
		}

		@Override
		public void doRenderLayer(EntityLivingBase e, float x, float y, float z,
				float p_177141_5_, float p_177141_6_, float p_177141_7_, float p_177141_8_) {

		}
	});
}

Oh My mistake, do i add it in the RenderPlayerEvent or in some other method somewhere?

He could so long as he records if it has been run and doesn't run again.  Silly way to do it though.

I would advise EntityConstruct event to add the your layer to the RenderPlayer instance.

Use Minecraft.getMinecraft().getRenderManager.getEntityRenderObject(EntityPlayerSP);

Make sure you use the client player, and check if the entity is instance of the EntityPlayer.