1. Buy an old mixer and track recorder from a garage sale or a pawn shop
2. Get microphones for the drummer and vocalist
3. Use the track recorder to separate the instruments so you can fine-tune the levels with the mixer
4. Record the tracks
5. Get a 1/8" - 1/4" inch female-male adapter plug and a male-male 1/8" cable, plug one end into the mixer's headphone jack, and the other into your computer's mic jack
6. Then record off of the mixer to your favorite audio program (audacity is the free one)
7. Master and add any effects you'd like, and render to MP3.
There are other ways, but that one is pretty simple, and renders decent results depending on your skill in recording and mastering.