I don't know why nobody seems to ever begin with: A filter is a voltage divider with frequency-dependent complex impedance.

If you can calculate what a resistor network does, or get a computer to do it, you can also calculate what a filter does (and you'll definitely want a computer for that).

(Going backwards, from a desired frequency response to a filter design, takes more learning. Luckily some very smart people already designed filter templates where you can just plug in the frequency you want)