Low-Pass filter on AVcc pin for ATtiny
I think the ATmega advice is pretty good.  With L1 = 10uH and C1 = 100nF, the 8MHz processor frequency is down over 60dB.  I added a series resistor R2 = 10&Omega; to reduce the resonant peak of the LC pair.  It may be not necessary, though.

Circuit







simulate this circuit &ndash; Schematic created using CircuitLab

Frequency response