Resampling is another case where there are multiple approaches and where there's multiple uses for a technique. Put simply, resampling means adding various effects and modulation to a particular sound or sounds, then rendering the audio out in order to repeat this process again, or use the rendered audio sample in the context of a track. As far back as the 90s, artists like Photek were using this process on drum breaks, however it has probably become most synonymous with modulated bass.
Below is my approach to getting a twisty, modulated bass sound. There isnt a right or wrong approach to resampling, and like most things in audio production, this is a skill which will impove over time and with practice. So, here goes.
1. Find or Make a Synth Patch
Decide on the sound you'll be using. Make sure to aim for something that will suit the context of your track. Obviously if you are using hollow sounding square and triangle waveforms or bright and aggressive sounding sawtooth waveforms you'll end up with a bass sound that sounds hollow or bright and aggressive. That may or may not suit your track, if your something soulful and warm then stay away form these waveforms and try others instead.
2. Make a Variation on Your Synth Patch
You should be able to do this fairly easily, just add some kind of modulation to your original patch, wether thats a moving filter, some distortion, or wavetable automation. You want to acheieve a variation thats similar in tone, but yet recognisably different so that you'll have lots of variation by the end point.
Record between 1 and 2 minutes of your synth patch, oscillating between the two variations you've made. You'll need a sizeable chink of audio for all the effects we'll be adding shortly.
4. Resample With Distortion
Apply some distortion. This can take a bunch of different forms. You could add three different distorion plugins, and three different saturation plugins and modulate the dry and wet amount of each, bringing them in at different times, there's no real rules to how you do this.
5. Pitch Up or Down
Try now pitching certain parts of your consolidated waveform up and down to see if you can get some interesting sonic results. If you want to remain chromatically accurate, then pitch up and down in octaves only. If you want more variation, use other intervals. You could also use a pitch shifter plugin at this point.
6. Resample With Time Based Effects
Bring out your favourite flanger, phaser and chorus plugins. Find some presets and tweak until you get some interesting textures happening. Again modulate the dry / wet balance in and out.
7. Resample With Filters
Apply a low pass filter to your audio, and modulate the cutoff point. You can experiment with layering filtered and unfiltered sounds together, resonance and other ideas.
8. Make Edits
Cut out all your favorite peieces of the resulting audio file, and of all of the audio files generated in steps 2 through 7 and beyond. Experiment with reversing certain parts of the regions you have at your disposal, and making other edits. This could include stretching, adding fades, whatever.
Consolidate your resulting audio into a new region, and put this in a software sampler. Now you have an interesting and highly modulated bass sample to play as if it were an instrument.
Find a couple of notes that sound really good together, and this can serve as an idea to construct a track around.