Generally a marketing consulting firm will work in a similar manner to a full agency. The steps that they would take rely heavily on the type of service or strategy you are looking for:
1) They make a proposed plan:
When you hire a marketing consulting firm, they will work with you to understand the product or service, its needs, then work with their internal teams to provide you with a detailed proposal. This proposal will include ideas, budgets, timeline and execution mediums. This plan is then used to finalize all the activities that would be undertaken by the consulting firm for the brand.
2) They work with externals:
More often than not, marketing consulting firms will strategize all the ideas, then work external teams and vendors to execute the activities. It depends on the brand or company and whether they want to hire the marketing firm for just strategy and planning, or if they would also like them to implement it. Usually companies hire the marketing firms for a full solution as that saves time and energy along with costs.
3) They evaluate & report:
A marketing consulting firm will always conduct an evaluation and continuously monitor their executed strategies to understand the success of the promotion. This is important for future strategies as well as changes in plans if needed. The consulting firm will evaluate their results, provide a full report to the company and modify their strategy if and when needed in the future.