Make an iCloud and/or iTunes backup of your iPad Pro.
1. Disable the Find My iPhone feature on the iPad Pro.
2. Disable every toggle in the iCloud settings on the device.
3. Sign out of iCloud and remove all iCloud info from the device when prompted.
4. Sign out of the iTunes & App Store.
5. Sign out of iMessage & FaceTime.
6. Delete all email accounts.
7. Login to sites like Dropbox, Evernote, Google, Microsoft, etc., if applicable, and remove the iPad Pro from your list of trusted devices.
8. Go to Settings→ General → Reset, and then select "Erase All Contents and Settings" or put the iPad Pro in DFU mode, connect it to your computer, and then select "Restore and Update" via iTunes when prompted.