In order to get started modding Day of Defeat, you will need some tools and files.
You will first need a map editor. Below is a list of map editors that you can use:
When Day of Defeat was popular, there was only one tool available, Hammer. This is/was the official tool for making maps. The last official "stable" version of the editor was version 3.4. Valve also release a 3.5 beta version of hammer as well. This version allows you to actually see the 3D models inside the editor. Unfortunately, the 3.5 beta is/was very buggy and would crash quite often. Which leads us to...
Yo Den took the last version of Hammer (version 3.5 beta) and modified it somehow providing a more stable editor with many bug fixes and new features.
Jackhammer is a brand new level editor for games with a quake-style BSP architecture. The aim is to develop a convenient cross-platform tool capable of incorporating the best features of existing editors, such as Valve Hammer Editor, Q3Radiant and others. Despite Quake and Half-Life having been released a long time ago, the large community have arisen around, still developing mods and games on their bases. However the existing editors suffer from fundamental disadvantages their users are well familiar with.
Sledge is a completely free and open-source replacement for Valve's Hammer Editor, with a focus on compatibility with both Goldsource and Source engines. The purpose of Sledge is to offer advanced editing features from later versions of Hammer and other editors and make them available to all users, regardless of which engine they choose to use.
Sledge uses the same file formats as Hammer does (RMF, MAP, VMF) and maps are compatible between both programs. The editing process is almost identical to Hammer, the hotkeys are the same, as well as many of the screen elements and dialogs. Switching to Sledge will not be difficult for users who are familiar with Hammer.