With the ascendance of the internet it is not really hard. There are services like toursbylocals. com (
Book Authentic Private Tours with Local Tour Guides | Guided Tours by ToursByLocals). Many of them have their own sites too, like
Tour Bulgaria with Patrick - the private guide. and yourowntour.it I prefer to deal direct with the guide candidates rather than through the filter of a booking site. I also like that all of the fee then goes to the guide.
I will start by searching for "private guide whatevercountry" That produces a lot of hits to sort through. My priority is to find an individual offering guiding, not a tour company. I then send inquiries describing our interest, our dates, etc. and read carefully what I get back. I want to sense that the guide is interested in customizing our time with them, not just running us through one of several canned itineraries. You can get a really good sense of a person in just a few exchanges and Zoom makes it even easier. I then ask for two or three references from people who have recently traveled with the guide; within the last three months for example. Everyone has a standard list of references they provide. That's not what I want.
Finally, I have a sort of travelers' resume that I send, describing us, our interests, our previous travel experiences, etc. One thing I specify is that we want older guides. The enthusiastic kids know the dates of all the paintings, royalty, and battles but do not have the life experience to talk to us about the country, its history, and current politics. The candidate guide's reaction to this is kind of the final test. If they come back having read it carefully and full of ideas, that's probably the winner.
I don't think we have ever had a dud. But so what if we do? It will work out. We had a guy in Ethiopa who showed up drunk for our first evening meeting at the hotel. Even that worked itself out and we had a good time overall.