Download a copy of the episode being fansubbed, which is referred to as the "raw." Your fansub group will generally have an FTP account they use to allow all members of the team to easily work on the project.
Download a copy of the translated script from your fansub group's FTP. It will often have been timed by this point, but not always, especially if the group's timer is out of pocket and the anime fansubbers do not wish to delay its release.
Open the script in Microsoft Notepad.
Open the raw episode in VLC Media Player or your media player of choice. Make sure that both programs are open side-by-side so you may refer to each of them simultaneously.
Watch the episode while referring to the translation on the same screen. Press pause at natural stopping points, such as the end of a line of dialogue.
Listen to the line of dialogue in Japanese while reading the English translation. While an editor does not need to be fluent in the language, he should be able to recognize common phrases and expressions, especially if the script is untimed.
Verify that the English translation for that line makes sense. Is there a better turn of phrase? Was one word used where a different word is preferable? Does the dialogue sound stiff and formal or does it read naturally and fluently? Make the appropriate corrections.
Repeat this for each line of dialogue until the script is complete.
Read through the script file. Correct typos that may have escaped the first edit. Check to make sure names are spelled the same way throughout. Is punctuation consistent? Are long lines of dialogue broken into segments that are natural and easy to read?
Rewatch the episode with the edited script alongside your video screen. Is the script as polished as you can make it? Make any last-minute changes.
Save your file, renaming the filename to indicate it has passed the first edit stage. Preserve any special filename extensions, such as .ssa or .eru, in your new file.
Upload your work to the group's FTP account so that the next person in line (either the timer or the typesetter) may do her job.