For me it would start from basics→ The check list. Something like below.

1. Is it a B2C or B2B firm? ( B2B firm can have better advantage with SEO )
2. What are they selling/ providing? and how?
3. Do they actually need SEO? If yes, How they can benefit from that? ( This one step might seems generic but most important )
4. The mediums they are using already to make the profit.
5. Scope of sell and growth, which will also include the ways in which we could move the strategy and how SEO can aid with that. The plan will also mention the existing negative traits followed by the customer that could hurt SEO. Essentially a self analysis report.
6. Once Scope is fixed, It is time to bring out the self analysis for one more time. The on-site technical and current SEO standing report. These types of reports will be iterative in nature, just to compare the growth.
7. Now on to an improvement plan for technical SEO is created.
8. Competitor analysis and keyword research follows.
9. Comparing the strategies used by competitors and easy keywords as a start. Do not target cardinal keywords as target in the first place.
10. Choose one the strategy that seems probable success. At this stage many people would suggest writing numerous blog posts and getting back-links. However, even with that your SEO might not yield results. So choose appropriate strategy. You can go through my previous answers to find about the strategies. Some case studies are also there. Quite useful for budding SEO superheros.