I suppose it depends on how much time/money you are willing to spend on it.
They make tape decks that are specifically designed to do this, and those are pretty much plug & play.
You can also do it with a male-t0-male stereo audio cable (probably 3.5mm, but check your equipment to make sure), your soundcard, a tape player that has a headphone jack, and
Audacity.
The second option is the least expensive, but the first option is likely going to give you the best quality.
Hope that helps!