The main things to keep in mind are that you're no longer rendering to the screen, but to the texture. This isn't much of a change, since when you render to the 'screen' you're really rendering to a texture as well.
Once you've rendered to the texture, bind it to a quad that's whatever size you want and render it like a normal polygon. You can then use a shader or d3d's built in functions to scale and rotate the quad with standard 3D matrices. if you don't rotate it around the Y or X axes, it'll remain a "2D" quad.
Just make sure you render the quad last with depth buffering disabled, that way it will always be on top of your screen.