A standard operating procedure is a written description of how a recurring task is performed.
What it is actually for
Delegation. A task that exists only in someone's head cannot be handed over without them standing next to the person doing it.
Consistency. Two people doing the same job differently produces two different customer experiences.
Improvement. A process that is written down can be examined and changed. One that is not is simply repeated.
Reducing dependence on any one person — see key person risk.
What makes one usable
Written by whoever actually does the job, not by whoever manages it.
Short. A procedure nobody reads has no effect, and length is the main reason they are not read.
Tested by having someone else follow it. Every gap the author assumed as obvious shows up immediately.
Where to start
The tasks done most often, and the tasks where mistakes cost most. For a trade business that usually means job completion, quoting, and how a new customer is set up.
Callback history is a good source of what needs one — see reducing callbacks.
Checklists rather than manuals
For skilled people the useful form is usually a checklist of the things that get missed, not an explanation of work they already know how to do.
Keeping them alive
A procedure that no longer matches how the work is done is worse than none, because it teaches the wrong thing to whoever is new — see employee onboarding.
Reviewing them when something goes wrong is the natural rhythm, and it is what turns a mistake into a change rather than a conversation.