Okay, first you need to open the RFA file in (I am assuming) RFA Explorer.
Open the file you want to edit, in your case for the Nitrous, ObjectSpawnTemplates.con. Make sure you are in the right folder, IE: /conquest/ObjectSpawnTemplates.con
Scroll down to the bottom of the file (it should be open in lower pane), and insert your code or codes.
Click the "Save" icons, and you should get that * thingy next to the file.
Now, on the left column click "Build RFA". You can save it over your current RFA file, or rename it with the next _00X filename, example:
You are editing Berlin_003.rfa, you can save it as Berlin_004.rfa. Make sure that if you are editing for BF1942, that you save it as _004 or higher. If you are editing for DC or DC Final, _001 or higer SHOULD do the trick. At least I hope so.
You can choose Compressed or Uncompressed, doesnt matter.
Click "GO" and it should save it out.
Now, optionally, if you are saving it as a one higher _00x.rfa file, you can delete all of the non-edited files in that additional RFA. Meaning, you can leave 003 or whatever alone, and then you just place the edited files in the new RFA. Make sure to keep the directory structure intact. Check for more on the _00x and related things.