If you're technically disinclined, you don't. The product as it exists relies entirely on the user to figure out how to make use of the RAM drive.
The simplest way to use the drive would be to install the game on it. The software supports automatic savign and reloading of the drive contents, so you wouldn't need to re-install the game or have a separate backup of any configuration files in the game directory. If the game is too large, then it gets more complicated.
One simple option is to use symbolic links to relocate certain critical files. Simple if you know what you're doing, anyway. Here are the basic steps to follow:
1) Figure out which files are frequently read by the game - maps, sounds, textures, etc. Prioritize and select the ones that will fit on the RAM drive.
2) Move the selected files to a backup directory, so their names are no longer present in the directory the game looks at.
3) Create the RAM drive and populate it with the chosen files.
4) Create symbolic links in the files' original locations and names to their locations on the RAM drive. Use the MKLINK command for this.
Now when the game wants to open those files, it will be accessing the RAM drive.
I just did a test with Crysis Warhead to see how it works in practice. The methodology was to simply time how long it took to load a particular saved game.
First, I did a quick and dirty cache flush by reading 16GB of unrelated data. The game is installed on a two-HDD stripe, and reads at around 220MB/sec on average. It took 50 seconds to load the save. Same result from two runs.
Next, I quit the game, restarted, and loaded the game again, without sucking up the system cache first. If you have a lot of RAM, Windows will have cached the files read by the game, so you'd expect this test to give better numbers. And it did - 26 seconds in both iterations.
Then I created a 4GB RAM drive and copied all of the PAK files in the Game directory to it, save the one for sounds (they won't all fit). I moved those files to a backup directory, and created a link to the copies on the RAM drive.
Before running the game, I swamped the system cache with 16GB of unrelated data again, to be consistent with the uncached HD test. It took 28 seconds to load the save in two iterations of the test. The game itself also took less time to reach the main menu.
Before you conclude that it's not worth it, you have to realize a couple things:
1) The cached game files will eventually be pushed out of memory by other things on the system. Copy files, encode a video, whatever - that will make the game load more slowly again. Putting that data on a RAM drive makes it a permanent cache that can't be cleared by other activity.
2) The system cache only captures what's already been read (and maybe a bit more with read-ahead). If you're moving from one map to another, with a completely different set of models and textures, the cache won't help you. If the data files are already on the RAM drive, the transition will be faster from the start.
It's still possible it wouldn't be much help with a 25GB game, depending on what the bulk of that space is used by. I'd have to see a directory tree listing to even guess whether it's practical.