Painting company. He uses his home office to talk to clients, create invoices and quotes. He store supplies in a different room. How do we take the home office deduction?

My husband owns an interior exterior painting and drywall company. He uses his home office to talk to clients, create invoices and quotes and manage his employees. He also uses a portion of our basement to store supplies and sundries and another portion of the basement for spraying (painting) cabinets when needed.

How do we take the home office deduction and what "percentage of time" should be used?