As Dinesh mentioned, it is easier said than done. However, it is not impossible. You mentioned that HR did not exist prior to your joining, which means you have to start from scratch. Therefore, manpower planning is very important. First, you need to have an organizational structure with a clear role for each position. Then you can consult with each departmental head about JD/KRA of their team members. Subsequently, KPIs can be defined.
And of course, at this stage, it would not be a wise idea to straightaway get into PMS without having the basics right. You can discuss with your management and do the PMS this year in the same way as they have been doing in the past. By next year, you can create a proper system for PMS, and that would be more effective.
Hope this will help you.
Regards,
Pramod