SOP (standard operating procedure)
An SOP, or standard operating procedure, is a written description of how a recurring task is done, so that it comes out the same way regardless of who does it or how busy the week is. For a small business its real value is not consistency for its own sake but that the task stops depending on one person's memory.
What a usable one looks like
Short enough to be read while doing the task, specific about what done means, and honest about the step everybody gets wrong. Long procedures written to be comprehensive are not read; the ones that survive fit on a page and name the decision points rather than every keystroke.
When to write one
Write it the second time you explain the same task to somebody, and rewrite it the first time it turns out to be wrong. Procedures written in advance for tasks nobody has done yet describe an imagined version of the work, and are usually wrong in the places that matter.