Step 1: Find 'GameConfig.txt' located in 'C:Program Files (x86)\Steam\steamapps\common\Portal 2\bin'

It will look like this:











Step 2: Open the file.

Step 3: Change all of the designations from 'C:Program Files (x86)\' to 'D:\' (or what ever drive you wish) There are 10 in all.

It should look like this:











That's it.

If you need to reset the destinations to there original settings, simply reinstall the program.

I hope this helps my fellow mappers.

I would like to take this opportunity to acknowledge tophattwaffle. Without his help, this guide would not have been possible.