You must be realistic otherwise the job description will look more like a process document. An example would be an Accounts Department, they would raise an invoice, email it to the client, but you don’t need to put in the job description when it is raised, how long you should wait for them to chase up for payment, how to allocate the payment to the invoice when it comes in. You just need to state they are responsible for raising the invoice and ensuring the payment is received.
Imagine if you documented all the things you did into every action, you would need reams of paper. Don’t get us wrong it is good to document processes so that everyone knows what to do, but that is another exercise. You may want to put it in the job description that they are responsible for documenting the process and review it every six months.
The main reason for having a job description is to make it clear what is expected of the employees, don’t over complicate it, but make it concise.
You will need to set targets and KPI’s. These differ from person, the time of the year, recent projects so these should be set in an appraisal or a one to one.