I have a confession to make: I haven't recorded a new voicemail greeting in nearly a decade. Since then, I've (hopefully) become more articulate, poised, and self-assured. But hear my voicemail recording, and you'd think I was still new to the work world, a little unsure of myself — and probably not an authority.
1. Write a list of information you want to include in your voicemail greeting. Writing out a list of points to include may sound arduous, but when your voicemail could potentially be the first impression someone has of you, it’s best if it doesn’t come with a bunch of "Um, uh," noises and awkward pauses. You want your voicemail to contain some basic information in a polite manner that will help the caller know that it’s you, and information to leave so you can call back as soon as possible. Include your name (and the company if this is a business voicemail), a statement that lets the caller know you apologize for missing the call, and information you would like from the caller such as name, number, and a brief message concerning the purpose of the call.

Website: https://intelligentoffice.com/blog/how-to-create-a-voicemail-greeting-for-your-business-including-scripts-you-can-use
Once you have your message, you need to actually record it. The exact process varies depending on whether you’re using a cell phone or office phone, but here’s the basic process: Press the voicemail button, or press and hold 1 on most cell phones. Enter your password. Record your message. Listen to the message you just recorded. Follow the prompts to save your message. Following Up on Voicemail Messages