You probably have an idea from your own experience (or from our other extremely useful sections) of what to look for in a good editor: Walter Murch.
Spoiler alert: You can’t afford him.
So that’s probably not going to happen, but at least you can find someone who’s organized and hard-working, with a grasp of narrative rhythm as well as software shortcuts. You’ll also want an editor whose style fits your story. Don’t get an editor who cuts hyperkinetic music videos if your project is a more deliberately paced character and conversation piece. The assistant editor might be a luxury for your production. But if there is room in the budget to fill this position, the assistant editor’s main goal is to make the editor’s life that much easier. Basically, the assistant editor needs to have the same skills as the main editor, but also the ability to forge a telepathic bond with the editor’s work habits and style. If that’s too much to ask, just focus on building a good collaborative relationship with your editor. Listen to whatever feedback is offered. Consider any suggestions. And don’t shatter the computer monitor in a fit of rage.