L4D Maps
How to install maps / mods for TF2.
Download the add-on.  - Step 1
All downloads are in .zip format. (You need to open the .zip archive to access the required files.)
If you can not open or extract the .zip/.rar file, download and install winrar for free.

How to install Maps.   - Step 2
Extract the map contents into your Team Fortress maps folder.
\Steam\steamapps\~username\team fortress 2\tf\maps\

How to install Mods.  - Step 2b
Extract the mod contents into your Team Fortress 2 folder.
\Steam\steamapps\~username\team fortress 2\

For additional support view the changelog or readme.txt associated with the download.

How to create a local server.  - Step 2b
You can join an existing server that already has the map or create your own server. (see below)