First, a disclaimer: I am not a lawyer. As with any company policy you implement, review by legal counsel is strongly advised.
Why do you need an acceptable use policy? It's not just a legal CYA, although it can serve that purpose. An acceptable use policy lets your employees know what your expectations are. Are you paying this person to work on your business, or surf the net for porn? Unfortunately, the most difficult part of implementing a use policy is getting people to follow it. If you have been operating without one for some time, habits and work patterns may well have been established that conflict with your new policy. Enforcing the policy requires that everyone in the company is committed to it, starting with the top management. "Do as I say, not as I do" just doesn't work.
While it is possible to create a policy that runs several pages, it's best to keep things brief:
The following policies for the use of computers and computer networks shall apply to all employees of Exampleco. Failure to comply with these policies may result in disciplinary action, termination, or legal action.