Just have to go to the relay manager, in their client, and save the relay settings in the appropriate event kind. All major clients have that, now.

Just go to https://metadata.nostr.com, or some other profile manager or onboarding client, and adjust the relays there. When they go to any other client, it'll pull that list. 

 That are still quite a lot of steps. The hardest thing to do is to add the npub on the whitelist of your relay on the fly imo