1) Prioritize Your Highest Impact Areas
Bring your team together, identify the core functions of your business (marketing, sales, operations, etc.), and take an inventory of your current strengths and weaknesses. Break each core function into sub-sections and prioritize your highest impact areas.
Note: You need to actually prioritize. You can't have 30 top "priorities." Create a high impact, but achievable, list of three to five processes to document. Systemization is a marathon, not a sprint -- starting small and building consistently is critical for success.
2) Select the Format & Outline Expectations
How much detail is appropriate for your team? Do team members prefer to watch how-to videos or read text? Who needs to participate in documentation? If a process needs to be improved, who is responsible?
Start by understanding your team's culture and choosing the appropriate format for them. Get your team's input, and make sure everyone is on the same page about documentation formats, how to follow processes, and who is involved in continuous improvement (hint: this should be everybody).
At GuavaBox, we use an ordered list of steps, add more details where necessary, and create a quick video overview so that team members can access bullet points for a quick reminder or watch the process being executed step-by-step if they need more detail.
3) Choose the Right Platform
There are tons of tools to choose from that are capable of storing your processes. But processes need to live where work is being done -- in other words, a process binder on the shelf that never gets opened is useless.
You could choose to build an internal site or wiki (using a platform like Google Sites or WordPress), you could rely on Google Docs or Evernote, or use a process documentation system like SweetProcess.
Ultimately, the tool is never as important as the actual acts of documentation and using those processes consistently.
Regardless of platform, what matters is understanding your needs, picking a platform that meets those requirements, and then committing to standardizing and scaling your agency.
I can't stress enough the importance of accountability to make sure your team members are all following the processes you've documented. If team members never referred to processes, then you'll continue to be plagued by inconsistent deliverables, spotty results, and inefficiency.