I only have a single (small) headphone output on the cassette player. Can I go headphone to RCA and then RCA to USB? My soundcard does not have an RCA input. Should I go through the phono amp or just directly from single headphone out to RCA and then use an RCA to USB cable?
If your cassette player only has a headphone jack, perhaps it would be wise to find (or borrow) a cassette player with proper inputs/outputs. The problem with the headphone jack is that it's controlled by the volume control. There's not an easy way to set it at the proper level. Do you even know if the headpone jack is stereo? (You didn't say.)
With that said...
To use the headphone jack, you simply need a patch cord with a 1/8" (3.5mm) stereo plug on each end. You would plug one end into the headphone jack on the cassette player and the other end into the blue Line-IN jack on your soundcard.