You must start the real client and have at least one running torrent on it first - use any torrent but the one you're about to fake on. A public one would be fine.
After that, run the RatioMaster, go to the Advanced tab, and select the emulation for the exact same version and build of the BitTorrent client you're running. (For this tutorial, we'll use uTorrent 1.8.2 Build 14458)
Afterwards, click the "Memory Reader" button:
Make sure the process name is correct. If you gave your client's executable a custom filename, you'll need to enter that minus ".exe" in the text field. The "String" box should already be filled with the chosen build's peer_id prefix. Click "Start Search". After the progress bar is filled with 2-3 blocks, this message box should pop up:
Click OK to use the peer_id, key, port and numwant extracted from the client's memory space:
Click Apply and then Close.
Note: this is the same tutorial you can find here with some fixes - the fact that it was buried on the mR thread made it difficult to find.
If you're running Azureus, you can use packet sniffing, or enable Logging and copy the peer_id and key from the announce dump.