I had the basics already implemented in my renderkernel, but I didn't actually have an implementation of the rendertarget interface (which in fact isn't anything else but a texture with a different type id (for me)). Internally (in the DirectX Resource Manangement of my engine) a rendertarget is automatically generated the first time the user set's the render target.
The interface is implemented in a way that I can use the same pointer to the object to set a texture.
Unfortunately there are almost no "old" sample applikations in the SDK that explain the usage of the device functions in detail. Some years ago (DX5, DX7) there was a quite good amount of samples that explained the basics. And today, if there's a sample, it's based upon the D3DX functions which are not usable, if you want to implement your engine in an API independant way... And it's hard to know what is "behind" those D3DX functions. I'ld be happy if Microsoft would hand out the source codes for the D3DX libs.
Oh well... after all I have my render to texture up and running, fixed all memory leaks and release problem (even found a memory leak in my player manager) and will go to bed... My daughter will get up in 4 hours and she'll surely come to wake *me* up...