You really cannot record in stereo
with a common cassette player.

They only have MONO/Single Channel Input.

You would have to use a multi-track or Stereo recorder.

You can create an effect which sort of "tricks the ear"

Use a Splitter Cable on Recorder’s Input,,
then use Dual Mic’s placed in different positions.

If They’re Far apart,,you create more of a reverb,or Delay Effect.

WHERE exactly to position each Mic depends on all sorts of variables.
But the Idea is to get Mic#1 recording a Bass-Biased Response,,and Mic#2 recording Hi Frequencies.

Though your Playback will STILL Be Only Single Channel,,
There will be more discrimination in the 2 different Frequency Ranges.
It gives a slight "audio illusion" of 2 diiferent Channels

IF You get Lucky.

It can just as easily sound pretty crappy too.

Recording in an area with lots of sound reflection aka "echo"
can produce same results.
Like in an average bathroom is a common example.
take down any shower curtains,,or hanging towels,,,
floor mats etc,,,open shower door,,,etc??

If you’re lucky,,,you can get a "faux stereo" playback sound.

Usually sounds better than straight,dry recording anyway.
………………………………………….
Here’s some quick examples

http://cgi.ebay.com/vintage-TEAC-CX-270-Stereo-cassette-recorder-Perfect_W0QQitemZ290048710945QQihZ019QQcategoryZ71574QQrdZ1QQcmdZViewItem

Note in the Description:

Left and right Record Volume controls

Tape counter

¼” front Stereo headphone jack

1/4" L and R Microphone jacks

L and R RCA Line-In jacks

L and R RCA Line-Out jacks

………………..
Here’s another(NOT WORKING),,,just for a look at Input Level Controls & Dual MIC Inputs(Left & Right)

http://cgi.ebay.com/TEAC-A-360S-Stereo-Cassette-Deck-Tape-Recorder_W0QQitemZ180048059081QQihZ008QQcategoryZ71574QQrdZ1QQcmdZViewItem

>>>>>>>>>>>>>>>
You can Only record in Stereo
by using a Stero Recorder,,
and 2 MICS.

Over Dubbing,,Layering tracks,etc,,,takes even yet More sophisticated equip.

Here’s one example,,,and sky’s the limit on price & feautures/capability

http://www.tascam.com/Products/MF-PO1.html

>>>>>>>>>>>>>>>>>>>>>
Hopefully someone who knows more about recording will offer more detailed specifics

Hope that helps!