Know what kind of editing you need
Proofreading.
Copyediting.
Line editing.
Structural editing.
Fact checking.
Style-guide compliance.
Define what should not change
Voice, terminology, formatting and technical language may need to remain consistent even while the text is improved.
Use a sample when appropriate
A short paid sample can help both sides understand the expected editing depth before a large project begins.
Editing depth is part of the scope.Two editors can quote the same manuscript while proposing very different levels of work.
Agree on file workflow
Clarify tracked changes, comments, clean files and whether multiple review rounds are included.