Adding Externally Generated Public Key to My Server

I'm trying to allow an external service to analyze project folders on my server. The external service has generated a public key that they have asked me to put on my server to allow them ssh access. It is a trusted source.

I have found a host of material on how to generate an ssh private public combo from within my server, but not how to implement one generated externally that must be copied into my own server.

What would the steps (and corresponding shell commands) be to put an externally generated public key on my own server which allows an external entity to ssh into my project folders.

I'm running ubuntu

1. Create a new user account for the remote user
2. In the newly created remote user's home directory create .ssh directory
3. Add the public key to .ssh directory
4. In .ssh create a file called authorized_keys
5. Concatenate the public key to .ssh/authorized_keys

After completing these steps, the external service with the corresponding private key will be able to ssh into your server.

Keep in mind, that they will have access to a shell and the filesystem. Set up all permissions before adding the public key.