Issue a contract
Always issue a contract, even it's just an email confirming rates and equipment
Once you're happy that the person you are talking to is right for the job, you can book them straight away, but usually productions can't make hard and fast bookings immediately, so pencil book the person and remember to call them back if you're confirming or cancelling. Once you have confirmed a booking you will need to issue a contract of engagement and purchase order.
In some cases, you will be booking crews 'last minute' and won't have time to issue a full formal contract before the shoot. Even so, you must at least send an email outlining what you have agreed. This is good for both you and the freelancer as it prevents any 'misunderstanding' which can be messy to sort out after the event.
As the shoot date approaches you'll also send them a call sheet and production risk assessment.