I have expertise and clients but no system. Where do I start?

Start where things are actually breaking, not at the beginning of a checklist.

Most people in this position have the same three leaks, in this order:

1. Enquiries go nowhere specific. They arrive across email, DMs and a form, and some are simply lost.
2. Follow-up depends on remembering. The ones that go quiet go quiet because nobody chased, not because the client decided no.
3. Delivery is rebuilt each time. Every client gets a bespoke process, which is why it never gets faster.

Fix them in that order. Fixing delivery while enquiries are being lost is optimising the part that already works.

What is actually breaking for you? Specific beats general here.