Seems I have to spell out how to use what is a very simple program so here goes.
Download the client here: https://www.evolvehq.com/client
Register an account, start the program, install any updates, and when the Evolve window pops up look in the top left corner for the "File" drop-down menu, click on it and select "Preferences..." now select the "Media" tab to the far right in the new window that pops up and adjust the recording settings to your liking.
I recommend Low encoding, Half-Size resolution and .mov format unless you have an editing program that handles .mkv, with these settings you shouldn't get much lag.
You may need to go to the "Features" tab in the "Preferences" window and tick the "Media" and "Game Overlay" checkboxes, you might also want to go to the "General" tab and un-check the "Launch Evolve on Windows start" option.
Check what hotkey you have set to start recording, it's under "Capture Hotkey" in the upper part of the Media window, the lower one is for screenshots.
To start record you need to have the Evolve client running, start Napoleonic Wars through steam or wherever you have it installed, when the game has started you can press the Record button to start recording and press it again to stop recording, from what I've seen you have unlimited recording time. Your media files will be saved to the folder you specified in your settings. Try out your settings on a decently populated public server before you start recording real linebattles and adjust them accordingly.
There are many more features to Evolve which you can read about on their website.