It's a difficult process if I remember, you need to get a .VMF from the map and a blank .VMF from Tf2 and try and splice them together using nothepad++ to make them compatible. I think I just copied the entities and brushes from the portal one and copied into the space in the tf2 one.
I'm not on my good pc right now, but i'll attempt to give a tutorial later if you're still in need of help