Account/Contact Roll Up Field help

I need to capture Orders from Contacts and store on the Account

Account field (Total Order) / Contact field (Orders Placed) 

Each time a Contact (all related contacts) has that field populated/updated - I want the Account field to continue to add them as a SUM

Ex. Contact 1 has 5 Orders / Contact 2 has 10 Order / Account should read 15 Orders

What is the best way to do this?