[synthsonix]

The key things to get this sound right -provided you get a decent sequence of notes in- is:

Glide between notes: quick glide, not too slow, but enough to get the notes slide between each other. That´s one of the key qualities of the 303 bass. Get a feel for the slide by playing it out with your hands, don´t jump to the sequencer.

Filter: Low Pass filter with A LOT of resonance. Then you play around with the Cutoff to morph the sound into the track.

Distortion: run it through some pretty warm distortion/saturation/wave shaper. It´s really important to get the huge resonant filter being heavily distorted to give it that 303 edge!