As maybe a more tedious example of an anime in which the OP isn't in a file seperated from the rest of the anime, and isn't always at the beginning of it, here's what I had to do for AIR
ffmpeg -ss 00:01:53 -i "[uguu~]_AIR_01v3_(BD-720p)_[9BC179D6].mkv" -t 00:01:33 -pix_fmt yuv420p -c:v libx264 -profile:v high -level:v 4.0 -map 0:a:0 -map 0:v:0 -movflags +faststart -vf scale=iw/2:ih/2 AIR_OP.mp4
For this one I included "-ss 00:01:53" at the beginning to indicate where I want to start encoding the video, and it's placement before inputting which file to choose is important as it allows the video to start encoding pretty fast. Only time you'd really want to put the "-ss" after is if it causes complications with the video, such as if there are softsubs you're trying to make hardsubbed into the video. The "-t 00:01:33" just indicates how long I want the video to be encoded from that earlier defined start point.
One thing I forgot to mention that's probably important is that to easily access the files without defining the folder I just shift+right-click in the folder with the files and click the option that says "Open PowerShell window here", if you're using Windows 7 and maybe lower it'll say something like "Open a command window here" and that'll work the same. Another thing I forgot to mention, the only other real variable in this is the final part of the line "AIR_OP.mp4" which you can probably guess what that's supposed to be, but I'll mention it anyways. It's the output file name and file type. Personally I prefer mp4 because it's easier to encode, smaller than webm, and looks better on average. Just as well, with webm you'd probably need to adjust more than just the scale on most videos to make the quality both not shit and not too big, so probably just stick to mp4.
Final note: The parts of the line of code I didn't mention are just so that the video is viewable on every browser, plays well enough on the server, and looks good. You'll never really change those, I don't.