Onboarding a software development company is similar to onboarding new staff. The general steps are:
Introduce your company;
Introduce the in-house staff relevant to the project;
Explain your policies, procedures, and company culture;
Explain your project, including the short-term and long-term goals;
Explain any prior issues or anticipated future problems;
Present the tools and software you use;
Provide the necessary technical documentation (codes, accesses, databases, development notes, etc.);
Clearly define the channels of communication you will be using.