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Read this book: https://www.amazon.com/Book-Estimating-Rehab-Costs-BiggerPockets/dp/0988973715
You can probably check it out from the library. You don't need all of it, just skip to the chapter on writing a Statement of Work. It will help you communicate with your contractor, and greatly reduce the chance of overages (but you should still plan for them of course).
Other than that, the best thing you can do is network a bit to find an awesome contractor.
I find The Book on Estimating Rehab Costs helpful.