His firm should be providing that information, if necessary, if he's with one, or if he's a solo his accountant should be guiding you with appropriate advice.

...how do you handle tax exemptions, etc?

My DH worked in CA earlier this year and they are asking him for tax info. including exemptions, etc. Is it best to withhold now and pay less later? Or not withhold at all and pay what you owe on the total billable hours you clocked in that state?

Thanks in advance for any advice.