In order for the project to be successful, you need to treat the consultant as the trusted partner that they now are. The consultant needs access to you, your board, your employees and information that they likely outlined in their proposal.
Provide information the consultant needs to get moving quickly and the staff hours necessary to accomplish the objectives. Help the consultant to understand your organization and what knowledge/information you have or are lacking.
Address any issues or misunderstandings as they arise. Be transparent about your issues, needs, strengths, and weaknesses so that the consultant can help you to develop a relevant plan.
Develop a mutually agreed-upon communications plan with meetings, reports, procedures for decision-making, and notification. Include regular check-ins. In your communication plans, be sure to cover relevant stakeholder groups including staff, board, and funders as appropriate.
Show up on time for meetings and try not to reschedule. You will set a good precedent for a respectful, professional relationship.
Keep staff and the board ad hoc committee involved in the implementation of the recommendations/plan. If you lose buy-in, not only will you lose the insight to improve upon the plan or project, you might lose momentum for the project altogether.