How do I create a simple client onboarding process?

Write down what should happen in the first two weeks, assign an owner and a deadline to each item, and do it the same way every time.

A workable minimum:

Day 0 — welcome email, deposit invoice, intake questionnaire, kickoff booked, client record created.
Days 1–3 — questionnaire chased, access requested, deposit confirmed.
Days 4–7 — kickoff meeting, definition of done agreed in writing, notes circulated within 24 hours.
Days 8–14 — something visible delivered, first status update sent.
Day 30 — ask directly whether the experience matches what they expected.

The single highest-value item is agreeing the definition of done in writing at kickoff. Most project conflict comes from two people holding different pictures of "finished".

Use the New Client Onboarding Checklist and the SOP: Onboarding a New Client together.