STEP 1: Install the Ongoing app onto your website.
STEP 2: Email Ongoing an exported CSV list of current subscribers from other Shopify app at support [at] ongoingsubscriptions [dot] [com]
Typically the list you export from the other app will have the following information:
Customer first name
Customer last name
Customer email
Subscription creation date
Subscription next billing date
Product name / price/ SKU
Subscription Status
Subscription Frequency (example: every 2 weeks)
Shipping Address
Shipping Price