All the SDK tools come with the game, so you don't need to install anything extra. (This also means you don't need to wait for something else to download!) The tools are located in the Steam\steamapps\common\Half-Life 2\bin folder. The folder is packed with applications, batch files, and DLLs, but there are a few key programs that we're interested in right now. In the bin folder, you will see Hammer, HLMV, and HLFacePoser. Hammer Editor is the tool you will most likely use; it is the application that you use to actually create the levels (maps). HLMV or Half-Life Model Viewer is a tool you can use to inspect game models in detail. HLFacePoser is used to sync lips to speech, and create custom scenes and NPC interactions.