You'll be opening Steam\steamapps\common\Half-Life 2\bin often, so you may want to create a desktop shortcut for easier access. Run Hammer.bat from the bin folder to launch Valve Hammer Editor, Valve's map (level) creator, so you can start creating a map. Hammer will prompt you to choose a game configuration, so choose which game you want to map for.

I will be using Half-Life 2: Episode Two in the following examples:

When you first open up Hammer, you will see a blank gray screen surrounded by a handful of menus, as shown in the following screenshot:

Like every other Microsoft Windows application, there is a main menu in the top-left corner of the screen that lets you open, save, or create a document. As far as Hammer is concerned, our documents are maps, and they are saved with the .vmf file extension. So let's open the File menu and load an example map so we can poke around a bit. Load the map titled Chapter2_Example.vmf.