Here are some basic tutorials for Native Instruments Reaktor. These noob starting points use the classic modular macro set. This set of modules is a great starting point to learn subtractive synthesis.
Using the classic modular macro set. This video shows you how to connect basic pitch and gate signals to oscillators. The p connection tells the oscillator which key you are pressing. the g tells the envelope when you have pressed, and then release the key.
This expands our setup to include a filter! The filter (low pass to start with) is fundamental part of any subtractive synthesiser. This scoops portions of the frequency range out of the signal. In this example the low pass filter attenuates the high frequencies (reduces them) leaving just the low frequencies. (bass)
This next tutorial builds a Reaktor FX instrument so you can process material on an audio track. If the concepts of Pitch and Gate, and the general operation of Reaktor programming are giving you a headache, then this is an excellent route to clearing out your misconceptions.
This video demonstrates the comb-filter as an audio process. (tuned short delays that create peaks and troughs in the spectrum that look a little like a comb.
We explore modulating the combfilter with both LFO and Envelope following.
LFO produces a cyclic repeating stead pattern at a specific rate and shape. The shapes correspond to the basic subtractive audio waveforms. A very common feature of synthesisers, samplers, and indeed simple effects pedals like tremolo.
The envelope follower is less common but possibly more useful. This produces a modulation shape based upon the incoming audio. You can shape the amount of time it ramps up and down again. Just the fact the movement is based upon what you play and how loud it is really provides a fundamentally powerful connection between your performance and modulation. Try modulating a low pass filter on a bass guitar. You get instant 70s funk.
This is a great way to explore, learn and identify different processes that could be useful in your creations. Just swap out the filter for other processes like the saturator and delay.