Once you have a shortlist of consultants and agencies, you can start approaching them. Many charities approach consultants by email or post, but calling them can be useful as it will give you a feel for how professional and responsive they are.
There are various options for what process to follow in choosing a consultancy.
The simplest process is simply meeting each of the consultants or agencies and having an informal discussion around what you require and asking them about their approach and their ideas for promoting your charity.
Some charities choose a more formal selection process, asking consultants to provide a proposal and then inviting all, or a selection, of the consultants to give a presentation or attend a meeting.
It can be useful to involve the trustees of your charity in the process, particularly if you have trustees with marketing or PR expertise.