offer to provide a risk assesment and method statement, show any qualifications if you have any, your insurance docs, ask them when the best time to carry out the clean would be to avoid disrupting the staff. be confident dont make out its your first office job.
before you look at the windows sit down and chat to the client first and get a lot of detail what they are looking for, one off, regular, listen to what they tell you and that can lead on to how your company can benefit them using points they bring up.
dont go in too cheap nofin worse than doing underpriced work