Funny enough the best way is to lead by example. I believe most if not all people can feel the difference when the dont just follow someone, but actually participate in something as an "equal" (whatever that actualle means in that specific situation).

This often happen organically in situations like disasters when people have to take responsibility and make decisions spontaneously jnstead of waiting for authorities to help them. In not so pressing situations its something that could be facilitated, but is pretty hard to do consistently.

Especially in conversations you might bring up examples on how leaderless / non hieraechical projects were successful and even bring up things in their own lives where they might already have leaderless / non hieraechical ways of doing things like friend / hobby groups etc.