Correct obvious spelling errors. This is already a matter of judgement. It should be possible to recover the original text just in case you or anybody else later disagree with yourself about the alleged error. Mark parts of speech. You definitely need explicit guidelines about which parts of speech are available and how to tell the difference between them.
Examples are essential but by no means sufficient.
Mark the boundaries of constituents, but don't give the constituents names. It will become obvious that you need an agreed mechanism for being vague about things that the annotator is genuinely unsure about. Mark full syntactic structure. In this case you need to convey to the annotators a common set of necessary and sufficient conditions for every possible construction. This is really hard.