Full disclosure, I am a contractor/consultant as a technical writer. My suggestions here are partly predicated on that fact. It's a mash up of how I wish clients came to us for work and how I prefer to find and hire my own contractors as an author.
1. Know what you want, when you want it, and why you want it.
I'm not trying to be glib. Knowing what you want determines who you hire. Knowing when you need it helps any potential contractors understand your time lines and the workload lift. Knowing WHY you want what you want goes to your brand. You and your contractor should know how the work you're contracting fits into your overall marketing/brand strategy. Value highly any contractor who can ask pointed questions about strategy and how what you're doing fits.
2. Have a minimum bar and do your research.
I have a minimum bar - I strongly prefer to work with contractors that come recommended by people I know. This isn't to say newbies don't stand a chance. It does mean that I will be more cautious working with someone who has no track record in publishing. But if they come in and do a great job with low drama, I will sing their praises in ALL the author spaces. This is what I mean by research. Look for people who do what you need, but vet them by asking other authors about the potential contractor. And when you find someone good, talk them up to your fellow authors. You'll find that even though your favorite contractor may get too busy or too pricey for you, they'll likely make recommendations from among their friends to help you get what you need, when you need it, at the price you can afford.
3. Consider a contract.
If you're looking to hire someone long term, consider writing up a contract. Preferably one that spells out your expectations of the contractor, what the contractor can expect from you, and what kinds of processes you'd like to put in place for managing disagreements, performance issues, or other drama that could arise. You aren't looking to spend thousands on a lawyer here. This is a basic tool created by collaboration and agreed upon mutually, then signed by both parties. Are there legal websites that will let you download a basic employment contract? Yes. It's a fine starting point. But unless it violates your minimum bar, you're not looking for a document that could be taken into court - you're creating a starting point for negotiation and conversation during a time when everyone might be stressed and not on their best behavior.
4. Good. Fast. Cheap. You can only pick two.
Do yourself and your contractors a favor. Ask about price up front. Yes you can negotiate to a point. But perhaps go to Tik Tok and find one of the maker channels playing "It costs that much cause it takes me fucking hours." Good help is worth every penny. Pinching pennies is likely to make more work than it saves you. Pay your contractors their worth or scale back your ambitions. There's no shame in having a budget.
5. Have a plan.
Have a plan for how this contract is going to go. Have a back up plan in case your contractor gets hit by a meteor. Have a plan for what happens if you get hit by a meteor. No one wants to live in worst-case-scenario-land, but you do have to plan for it. When things go well, you can breathe a sigh of relief and file your emergency plans away for another day. If things go to hell in a hand basket, however, you'll have road map to help you navigate while you're in the middle of freaking out.
Last but not least:
6. Don't forget the taxes.
Depending on what you're doing/having done, you may be liable for supplying a 1099 to your contractor(s). Make sure you cover that base with a tax professional. No one but no one wants the IRS mad at them. Just saying.