How do I design a low-pass or high-pass filter?

If you're trying to design a low-pass or high-pass filter, I’d recommend starting with the RC (Resistor-Capacitor) circuits — they’re the simplest.

For a low-pass filter, I usually connect a resistor in series with the input signal and a capacitor to ground. The capacitor "shorts" high frequencies, allowing only low frequencies to pass.
The cutoff frequency (fc) is calculated using the formula:
fc = 1 / (2πRC)

For a high-pass filter, just swap the resistor and capacitor. This setup blocks low frequencies and lets the high ones go through.

If you’re working with op-amps or want active filters, then the design gets more interesting. But for basic filtering, RC circuits are a great starting point. I used them a lot during my early projects!