1. Open your Steam Library
2. Choose tools and download Source SDK
3. After the download you open your Steam Library again ( Make sure your game is closed )
4. Choose a game you want to create a map to and right click on it and choose 'Properties' ( Only tf2, dods and css )
5. Click at the tab called 'BETAS'
6. In the dropdown list you choose 'sdktools_beta - Includes files needed to make it...' and click 'Close' ( If you can't choose this one in the dropdown list then you might have to restart Steam or your Computer )
7. Now your game will download few MegaBytes
8. Go into the folder C:\Program Files (x86)\Steam\steamapps\common\
9. Open the game folder for the game you chose ( tf2, dods, css ) and inside that folder you open the folder called 'Bin'
10. In this folder there's a file called 'hammer.exe' and when you open this file you have to choose which game you want to create a map for ( tf2, dods, css ) and after you have clicked on the game you want the map for and have clicked 'Ok' the Hammer Editor will open and you are ready to build your map.
I recommend that you create a shortcut at your desktop to open this 'hammer.exe' file