I'm going through a similar process at the moment with one of my business.

The quickest and simplest way I've found is by offering your service for free to a handful or people in your existing network.

Explain to them that you are starting out with your coaching + consultancy business, explain the value and the help that you are able to provide with your service and that you are happy to provide this service to them for free for a period of time in exchange for them providing you with a testimonial at the end of this period of time with feedback on their experience of your service.

These testimonials can then be used in your marketing for paid clients and you will also gain hands-on experience while you are providing free advice, which means you can test ideas, strategies, tactics, tools etc, and be able to offer a better service to paying clients - which feeds the cycle of testimonials and social proof, which will benefit your business and service in the long run.

Making the investment in time and effort upfront will pay off in the long run and you have to be patient with this approach. The good thing is, because you are offering a service for free to your existing network should present you with plenty of opportunities to develop your service offering for paid clients and find willing takers for your free service.

Hope you find this useful and if you want to talk through specific strategies and tactics that you can use I'd be happy to jump on a call and help.

Good luck!