Humans are usually bad at randomness and good at repeating stuff, so randomly picking words from your brain or some list is not really random.
Easiest way would be using dices or cards, but I saw some crazy stuff people are doing with Geiger's counter recently, because radioactive particles are random.
This is fun and extreme geek tech using python 3 script generating BIP39 mnemonic codes with random data coming from a Geiger counter, and other guy even added RaspberyPi, Audio interface and Americium 241 from a Smoke Detector 

https://twitter.com/danieldemercado/status/1437033277358022657?s=21

PS
My point is that randomness is not easy like it seems, but you shouldn't use Geiger's counter for generating your bitcoin wallet if you are a newbie.