In your Dashboard, click Add Subgraph button on the right side. Fill in the specified mandatory fields, which are subgraph name and subtitle. Select your github account.
When you will be filling subgraph name, you can see the URL which will be used for it in future, something like thegraph.com/explorer/<github_name>/<subgraph_name>.
Click Create subgraph button. Then you will see it in your Dashboard.