Half-Life stores the majority of it's content in PAK files. The PAK file is an archive that contains files in a specific directory structure that the game can read from.
For editing purposes, it is most preferable when some or all of a PAK's contents is "unpacked" - that is, that the contents reside in their normal directory structure rather than inside the PAK file.
There are a number of benefits to having the PAK file.
easy access to sounds, sprites, and model files.
The first thing you need is a program capable of browsing and editing the PAK file. Wally is a good tool for this, since it's also a fantastic texture editor and is freely available. You can get Wally from it's website, here -
Unpacking PAK contents is pretty simple. The following example shows how to unpack the Sprite folder.
Load Wally and click on Open in
the File menu. Browse to the Half-Life folder. Double-click on the
valve folder. Click on pak0.pak and press the Open button.
It's important to maintain the correct directory structure of files. If you're unpacking a PAK that is in a specific game directory, the unpacked files should be exported only to that game directory.