Go to 'Settings -> Integrations'. Select Slack from the list of integrations.
Click the "Connect to service' button. This will open Slack. Enter your credentials to log in to Slack. The next step you will choose which channel that MakePlans will post messages to.
You will then return to MakePlans to select which MakePlans account to connect to. Select your account.
That's it. MakePlans will now automatically send a message to your Slack channel whenever a new booking is made.