From SimSynth's presets list, choose 'BLANK'. This will strip the synth back to its most basic setting of a single oscillator. The sound is very basic but don't worry'"that's the point.
You can see three oscillators but at the moment only one is active, shown by the light on its left hand side. There are five waveforms available so audition them while holding a note. Here I've gone for the Saw wave. Then, click the '1' button to add a second tone one octave up and fatten up the sound.
Moving down to oscillator 2, switch it on using the yellow button and then make settings in the way you did in the previous step. Choose a different waveshape and this time, use the Coarse tuning knob to tune the second oscillator up several semitones. Five works well, since this creates a thicker sound but one that's still harmonically pleasing.
Now bring in oscillator 3 and this time, select a noise source so you add some grit to the signal. Use the level knobs on each oscillator to balance them against each other and see what works best. If you activate the 'warm' button for any oscillator you can bring some unison effect to it, creating a wider sound.
The LFO section at the bottom right-hand corner also has a selection of shapes it can use, as well as a rate and delay control. The amount of LFO applied to each of the oscillators is determined using the LFO dial on the oscillator itself, so you can send it in differing amounts to each one, or indeed send none at all to certain oscillators. Adding some LFO to one or more oscillators can give a more unpredictable character to the sound by making it move and sway a little.
The SFV section is used to apply filtering to the signal and you can turn it off if you like, or use the cutoff and resonance knobs to filter the sound, and the ADSR controls along its top edge to control the way the filtering comes in, sustains and releases. It's also possible to make the filter modulate based on the LFO settings from the previous step.
Finally, using the Amp section at the top right you can control the ADSR of the whole signal. Try moving the attack knob a little to make the sound come in more gradually, and the release control to make it take a second or so to fade out. Or, reverse the settings for a very short, sharp sound.