shooting at dusk

Wait until the sun goes down. Wait longer until the sky goes darker.

Shoot for 15 minutes. Go home before it's dark. Oh, and use a tripod. 

Please.

portraits in window light

Shooting a window light portrait is easy. Put your subject a foot away from the window. It makes a soft directional light. You might need a higher ISO such as 400 or 800.  Get close with your telephoto. Bounce your flash off the wall on the left so that it fills in the shadow. Or use a reflector. Or try with no fill but the shadow side will be fairly dark, for a more sombre portrait.