Implementing efficient and precise performance management system in a small business?

I need some guidance and help in structuring performance review systems in a small digital advertising startup of 25 people having 4 different departments. How can I help in organizational development by implementing efficient and precise performance management system and is it important to automate it?

Keep it simple. Match the KRAs and Job responsibilities. Rate the employee with the help of the respective manager.
Pen down the areas of improvement & new areas to explore in the upcoming months and share with the employee.