Instructions
Download and install Tortise SVN. SVN stands for "subversion" and is a program for synchronizing files. Wiremod uses SVN to keep all users of Wiremod up to date with the latest version.
Navigate to the "addons" folder in your Garry's Mod installation and create a folder called "wiremodCO" to use with SVN. Right-click on the folder you just created and select "SVN Checkout" from the drop-down menu.
Enter "https://wiremod.svn.sourceforge.net/svnroot/wiremod/trunk/" in the field marked "URL of repository" and click "OK" to begin. This will download all the necessary files for Wiremod into the folder.
Copy all of the subfolders out of the "wiremodCO" folder and paste them under "addons." You can now delete the "wiremodCO" folder, since Tortoise will remember where the contents were placed and be able to update them.
Start Garry's Mod in single-player mode. You now have Wiremod installed, and can find all the tools under the "Wire" tab of the build menu. Access the build menu by holding down the "Q" button when in-game.