As a business consultant and specialist in simplicity/productivity, I've worked with many fast-growing clients to create SOPs.
1. Why SOPs:
If operations and knowledge aren't written down, then they will live in the heads of a few, key employees. This results in those employees being overworked while other employees are underutilized due to lack of knowledge.
SOPs should first be used to create those processes which are used 80% of the time during the workday. After that, the less common (usually more complex) tasks can be written down as they occur.
3. How To Get Started:
Have the person who is currently knowledgable of the task write down the steps as they are actually performing the task. This way, they are less likely to leave out any steps. Then, the next time the same task needs to be done, a new person should do the task while following the SOP, with the expert observing. This will help them streamline the process and discover any missing steps.
One important thing about SOPs, that I have learned from some of my clients, is that the SOP should be written simply, and be easily accessible to all necessary employees.
I once had a bank as a client who recorded SOPs in a formal, wordy format (with unnecessary details) and stored it on a crowded internal network. People didn't use it because it was much easier to ask people informally than to locate and read through the entire document.