(FWIW, I didn't really follow all of these steps in writing Cloelia. I am trying to save you woes and rewrites by suggesting a better way of doing things.)
More under the cut.
Before you ask for help:
Be your own harshest editor, as much as you can. See the previous post for how to do that.
Keep your goals in mind: it’s easy, especially if you’re writing in the mythological realm, to end up going down a rabbithole or adding a lot of unnecessary details. Keep your audience’s proficiency level and the themes you want to focus on in mind. No one likes a rambler. For this reason, I strongly suggest having a separate brainstorming document where you let your imagination run wild. "what if she did this and then she can meet Theseus and do they wrestle or is it here and Peleus and oh crap was Theseus king yet?" that kind of thing. You can also use this document to work out themes and issues you want to focus on in advance.
Write a lot of it first, ideally a full draft (a bad one obviously is fine) and outline, so that pre-readers can sit down & deal with your work in a day or two, instead of going chapter by chapter as you write it. This way is more respectful of their time, and more productive in terms of fixing plot holes, finding what expressions you need to use over & over, etc. Also, frankly, it means you can be more creative because no one's stomping on your ideas right at the beginning.
Asking for help, part I: people you know
And hey, why not ask your actual audience-- your students? They will be super impressed that you’re writing “a whole book” in Latin, and they also won’t pull any punches when it comes to what’s confusing or what parts of the plot are boring. Your students are your best bet for finding out what’s compelling.
Use what you learn from this pre-readers to improve the plot, the characters, etc. The more of the final product you can get done before asking non-friends for help, the better.
Asking for help, part II: people you maybe don't know
Finally, once you feel like you have your story down and the writing is a rough draft, politely ask several really good Latinists- you know who the best Latinists in your circles are- to read it over for you and mark errors.
It's a good idea to ask ahead of time what kind of credit they want to get for their help, too.
You do want multiple editors if possible because one of the weirdnesses of working in a dead language is that you can have two or more very different ideas of what constitutes “real” Latin even if all parties are equally proficient (which they aren’t).
If they say they can't, don't take it personally.
If they accept, thank them profusely, then wait PATIENTLY for your responses. They are busy. You are also busy, so you know. Editing someone else's Latin is also really tedious. You have to look up everything you're not quite sure of, even if you're pretty sure it's wrong-- what if this person just happens to know something you don't? It takes a lot of time and effort. Please respect that time and effort.
Take their advice. If you have questions, look it up in the tools in the previous post. If you still have questions, ask the editor. Try to get all your questions together and ask them at once lest you take up more of their time than necessary. But if an accomplished Latinist tells you something is wrong, and you look it up on your own and they are correct, you should probably listen. They're not doing it to be nasty. A good editor will also make a distinction between things they don’t like versus things that are actually wrong.
One thing I didn't understand til a conversation recently: if a Latinist, let's call her Erasma, gives you advice, and you don't take it, but still put Erasma in your acknowledgements, it makes it look like Erasma has stamped approval on your errors. This impression could impact Erasma's reputation as a Latinist, and if she's an academic, maybe even her career prospects. That's serious business. Respect that. I don’t know if this has happened; I just know it’s something some people are concerned about, so bear it in mind when you ask for help. I also totally agree that it’s ridiculous to draw that conclusion, but hey, welcome to academia. ‘Tis a silly place.
Also, there will always be spots where it's actually harder for the editor to explain what's wrong than to just rewrite the sentence. And if that happens over and over again, at what point do they become the co-author? You have to do your due diligence up front so that you're not asking them to write your book for you. A good editor will, of course, explain what’s wrong. If you find that your editor tends to rewrite things for you, let them know you’re concerned that you’re asking too much of them and maybe try to steer them gently toward the “did you check the usage of habere with that noun?” level of advice. If they rewrite things for you and don’t explain why, ask why.
Overall, be patient with yourself and others. There's no need to publish your novella right now even if you're super excited. Publish it online, by all means, and get lots of feedback, but maybe hold off on the paperback. It's better to publish something polished and not have to make too many fixes than end up making multiple new editions because you didn't listen to someone the first time they told you how inquit works, not that I am talking about anyone specific here and definitely not about myself. Ahem. Apropos of nothing, Cloelia v.1.2 is in the works.