How do I write a cover letter for a business consultancy proposal

The first thing that I would suggest doing is performing a Google search in how to write a cover letter.

How To Write a Cover Letter - Google Search

We can't actually write your cover letter for you here, but if you would like to post a draft of your letter, then I'm sure that there would be those of us who would come along and be willing to take a look at it in order to critique it and advise you as to how you might make it better.

Thanks!