Communication is key — you've got to transfer a lot of institutional knowledge, and give the new designer the tools to start contributing quickly. This is especially important when growing from a design team of one, since you may be codifying parts of your process for the first time.
1. Document and collect important information in one place.
Design research, customer interviews, process outlines — all this should be collected in Google Docs, Dropbox Paper, or something similarly accessible. If your team is very small, a lot of this information may not be documented consistently. Get it all down in writing, so new team members can catch up quickly.
2. Walk the new designer through your team's process.
Explain each step of your process, detailing both the what and the why. Describe which parts are set in stone and which are flexible, and ideally have your new teammate shadow an existing designer as they go through the process on a real task.
3. Give your new teammate some quick wins.
Give your teammate the tools to start contributing quickly, so they feel like a valuable member of the team and not just "dead weight" until they're fully onboarded. Give them some smaller design tasks that they can help with right away, and help walk them through the team's design process if they get stuck.
4. Ensure the new designer has contact with the rest of the organization.
Introduce them to all the stakeholders and points-of-contact they'll be working with throughout the organization. This ensures that (a) they'll be familiar with the people they'll be talking to, and (b) makes them more approachable to sales, engineering, customer success, and other teams throughout the company.