Step 1. Use oDesk.com or a similar online offering.
Step 2. Post a great job description. Ask respondents to answer specific questions. If real-time communication is important, specify the timezone.
Step 3. Shortlist applicants.
Step 4. Hire initially on a short term basis as a trial
Use Elance.com, Higher the assistant per hour not per project to avoid future conflicts if you have some extra work more than what's submitted in the project details, also higher a company not individuals so they will have more than one expert to replace the first one in case of emergency.