A process that only lives in one person's head isn't a process, it's a single point of failure with a name attached. Writing it down is what actually lets a team scale past you.
It's easy to resist SOPs early on. They feel like bureaucracy for a team small enough that everyone just knows how things work. That works right up until it doesn't, until the one person who knows the process is out sick, or the team doubles and nobody can repeat the same explanation a sixth time without losing an afternoon to it.
SOPs aren't set in stone. The good ones are living documents that evolve as the process does, not a document written once and forgotten in a drive somewhere. The point isn't to freeze the process, it's to make sure the current version of it lives somewhere other than one person's memory.
Ask yourself: are you repeating the same explanation to new team members? Whatever answer comes to mind first is the process to document this week.