You need to import the contacts, then send them a message template (you can set one up on Settings > Whatsapp > Message Templates).

These are paid messages.

Also make sure you send less than your maximum Business Initiated Conversations per day - which is at Settings > Whatsapp. This usually starts at 250 for unverified accounts and 1000 messages for verified accounts.

As for best practices: don't spam people :D

thanks indeed