Please explain the procedure of getting disassociated from I CLOUD

To back up your data plug in your external hard drive, open time machine from system preferences and choose the drive as your backup destination; you can flag automatic backups too if you prefer and whenever you'll plug in the external drive your backup will be refreshed without the need of a command. If it's a new drive you'll have to format it with disk utility before use.

If you want to disassociate your Mac from your iCloud account you can do it from icloud.com, but unlike with iPhones, data of the Mac is not backed up on the cloud automatically.