In a nutshell, create a drip campaign that triggers when a contact is pulled into a custom list that collects members who match a specific deal stage.
Create a single value picklist custom field on a contact that will hold a deal stage value.
Administration->Account Settings->Custom Fields [Add Custom Field Type]
Be sure to list the deal stage IDs in the options, one per line.
If you have not done so, set up a new import scheduler.
Contacts->CRM Contact Updates [New]
In your import scheduler, map deal stage to your custom field.
Set up a custom dynamic audience (list) that collects members based on your custom field (deal stage). (Contacts->Contact Lists)
Create a new drip campaign (Automation->Campaigns->Drip Campaign
Select the 'Added To List' option
Select your starting step, choose your list