Converting Cassettes is a bit of a pain depending on how they are organized. If you are lucky and have only one lecture on each casssette, you could convert each cassette into one "audio book" and label them distinctly so that someone could go to individual lectures. I.E. Milton in front of:
On the Morning of Christ's Nativity
A Paraphrase on Psalm 114
Psalm 136
The Passion
On Time
At a Solemn Musick
An Epitaph on the Marchioness of Winchester
Song On May Morning
On Shakespear 1630
That is, each would be similar to Milton - The Passion
or Milton, lecture 1 of 23, Milton 2 of 23, etc
or Milton, 1_23, Milton 2_23, etc
If the cassettes are not separate lectures, you will have to listen to the recording and manage manually or use some of the techniques discussed in the guide to locate and split the recorded file into multiple files using Audacity, Total Recorder, or Polderbits -- I forget which one allows creating files from segments of the overall recording. Check the guides for which one does that.
Use a stereo component cassette deck wired to the in jack of your sound card. The links above have very specific information on how to wire and record cassettes to iPod audio files as does the signature below (same thread, direct to Cassette post with Cassette link in signature below). My guide anticipates converting true audio book cassettes into as few files as possible using an automatic play deck which plays cassette 1 side A then side B and then cassette 2, side A and then side B, etc. (5 hours max unless recent software releases have fixed the bugs), but you should be able to adapt it to your purposes.
Best of luck.