Here is the short guide written 1 year ago.
Change synchronization settings to SharePoint Active Directory Import
Check “Profile Synchronization status” to be “Idle”
If “Profile Synchronization status” is “Not provisioned”, go to “Services on server” and start “User Profile Synchronization Service” service
Enter user name and password, check for timer job “ProfileSynchronizationSetupJob” execution in “Monitoring” – “Timer job status”
- Go to “Synchronization Connections”, create new connection
Enter in “Fully Qualified Domain Name (e.g. contoso.com)” full domain name, e.g. (1 domain name for each connection):
XXXX.XXXX.com
Choose “Authentication Provider Type”
Enter AD sync account login\password
Set check-box “Filter out disabled users”
Click “Populate Containers”
Select required containers (for example):
CN=Users,DC=XXXXX,DC=XXXXX,DC=com
OU=Users,OU=XXXXX,OU=BO Users,DC=XXXXX,DC=XXXXX,DC=com
After all domains are added, check for “Profile Synchronization status” to be “Synchronizing” and “User Profile Service Application - User Profile ActiveDirectory Import Job” to be running
Constantly monitor the import status by checking /_layouts/15/ManageUserProfileServiceApplication.aspx?%ApplicationID% page in CA
In case of errors, check ULS logs