The process is a little different for each person and each country. The biggest part is a three-step authentication for your diploma, police check (and TEFL certificate for a number of the areas).
It is a little confusing but basically before the Chinese embassy (often done through the visa office) will authenticate the documents they need the documents notarized by the state/province/county, and before the state/province/county will notarize/authenticate the documents they need to be authenticated locally.
There are companies ($$) that can help with the process as well depending on how far teachers are from the services they need.
Please click here for the document list that you need to prepare for us to process the work permit for you. (The work permit is needed before you can get a visa)