Click on this link https://docs.microsoft.com/en-us/azure/azure-resource-manager/management/tag-policies and select the required tag policies

In our example lets see how to assign tags for resources

Click on the Add a tag to resources

Below policy will be opened,

Click on assign and select the desired subscription where you want to add the tags, Optionally you can also select the resource group

you can also specify exclusions.

In next tab enter the tag name and value

By default, this assignment will only take effect on newly created resources. Existing resources can be updated via a remediation task after the policy is assigned. For deployIfNotExists policies, the remediation task will deploy the specified template. For modify policies, the remediation task will edit tags on the existing resources.

Verify the assignment under policies

For resources which are already deployed, click the remediation task as shown below and wait for it to complete

Now the below task is completed and it says there are 53 resources to remediate

Click on remediate

Task completed

Now lets check the existing resources and see if the tags are applied

Existing resources are also applied with the tags which we just created!