From Valve Developer Community
- Within Steam under Library then Tools run "Dota 2 Workshop Tools"
- Click Create Empty Addon and name it something appropriate. For this example it will be "myaddon"
- The new addon will become highlighted and marked as default
- Click Launch Dota 2 Workshop Tools
- In the Asset Browser window click on the Hammer icon
Note:Hammer requires Dota 2 to be running. If you close Dota 2 all the associated tools will also shut down.