Hey, Anon! This is admin ʀ~
I’ll split this answer into two parts since there are two things being asked here:1. How do I download videos from NND?2. How do I encode a video for upload on NND?
Both questions require a lengthy answer. I’ve tried to keep it as simple as possible but still, I apologize for the huge chunks of text. I know it can be quite difficult to read but I hope you can bear with me;;;

★How do I download videos from NND?
Although you may not like the Firefox add-on for downloading videos off of NND, I will have to suggest it here anyway because in my experience, NicoFox has been efficient and reliable. It’s the most raw you can get because it downloads the video in the original format it was uploaded in. I rarely encounter problems with this plugin and I find it really easy to use, so I highly recommend it.
If that really isn’t your cup of tea, then an alternative to would be to get Craving Explorer, which is a special browser (like Firefox, Google Chrome, etc.) that allows you to download videos from YouTube and NND. You can download the browser here.
Craving Explorer allows you to choose which format you want the video to be downloaded in. The browser automatically converts the raw file into whatever format you want it to be. As useful as that sounds, it can actually be a problem because the browser’s converter is not the greatest and sometimes, you’re left with a video that doesn’t encode properly when rendering your cover. Also, the browser is in Japanese so it may be a little difficult to use for those who can’t understand.
One thing to note though: both cannot download in high quality during NND’s economy mode. You’ll either have to wait for economy mode to go away or ask someone with a premium account to download the video for you since premium users have HQ on for the entire day.
★How do you encode a video for upload on NND?
It really doesn’t matter what converter, encoder or software you use to make your video NND-friendly because what is most important is that the video is rendered with the settings that NND accepts.
Some people use actual software like Camtasia, Sony Vegas or VirtualDub, others may render it in Windows Movie Maker and then use an online converter. Whatever means you use will be alright as long as your desired result is attained.
Uploading videos on NND (Free users):
Accepted formats: FLV/MP4/DivX/XviD/MPEG1/MPEG2/WMV
Available space on account: 2 GB
Max file size per upload: FLV ~40MB; MP4 ~40MB; Other formats ~100MB
Highest accepted bitrate for FLV & MP4: 600kbps*
Accepted dimensions: Within 800x600
Dimensions for best resolution: 512x384 (4:3) and 640x384 (16:9)
* If you want really good quality audio, the bitrate of the video should be at least 400kbps
As long as you encode or render your video with the settings stated above, it should appear fine on NND without any distortions. You’ll have to play around until you get the quality you want. Just use the points above as reference. It’s all about trial and error.
Hope this helps~admin ʀ
—
Hello anon! Admin G here to add to what admin ʀ wrote.
By experience, you don’t need to hit all those parameters above so don’t worry about missing one or two of them for the perfect encoding. Not all programs come with such detailed encoding settings but try to hit it close.
Remember to check the aspect ratios (go with Square) and size of the video though (convert the video size if you have to). If they’re not correct you will get those dreaded black lines on the sides of your NND video.
Good luck uploading! 

Hey, Anon! This is admin ʀ~

I’ll split this answer into two parts since there are two things being asked here:
1. How do I download videos from NND?
2. How do I encode a video for upload on NND?

Both questions require a lengthy answer. I’ve tried to keep it as simple as possible but still, I apologize for the huge chunks of text. I know it can be quite difficult to read but I hope you can bear with me;;;

★How do I download videos from NND?

Although you may not like the Firefox add-on for downloading videos off of NND, I will have to suggest it here anyway because in my experience, NicoFox has been efficient and reliable. It’s the most raw you can get because it downloads the video in the original format it was uploaded in. I rarely encounter problems with this plugin and I find it really easy to use, so I highly recommend it.

If that really isn’t your cup of tea, then an alternative to would be to get Craving Explorer, which is a special browser (like Firefox, Google Chrome, etc.) that allows you to download videos from YouTube and NND. You can download the browser here.

Craving Explorer allows you to choose which format you want the video to be downloaded in. The browser automatically converts the raw file into whatever format you want it to be. As useful as that sounds, it can actually be a problem because the browser’s converter is not the greatest and sometimes, you’re left with a video that doesn’t encode properly when rendering your cover. Also, the browser is in Japanese so it may be a little difficult to use for those who can’t understand.

One thing to note though: both cannot download in high quality during NND’s economy mode. You’ll either have to wait for economy mode to go away or ask someone with a premium account to download the video for you since premium users have HQ on for the entire day.

★How do you encode a video for upload on NND?

It really doesn’t matter what converter, encoder or software you use to make your video NND-friendly because what is most important is that the video is rendered with the settings that NND accepts.

Some people use actual software like Camtasia, Sony Vegas or VirtualDub, others may render it in Windows Movie Maker and then use an online converter. Whatever means you use will be alright as long as your desired result is attained.

Uploading videos on NND (Free users):

  • Accepted formats: FLV/MP4/DivX/XviD/MPEG1/MPEG2/WMV
  • Available space on account: 2 GB
  • Max file size per upload: FLV ~40MB; MP4 ~40MB; Other formats ~100MB
  • Highest accepted bitrate for FLV & MP4: 600kbps*
  • Accepted dimensions: Within 800x600
  • Dimensions for best resolution: 512x384 (4:3) and 640x384 (16:9)

* If you want really good quality audio, the bitrate of the video should be at least 400kbps

As long as you encode or render your video with the settings stated above, it should appear fine on NND without any distortions. You’ll have to play around until you get the quality you want. Just use the points above as reference. It’s all about trial and error.

Hope this helps~
admin ʀ

Hello anon! Admin G here to add to what admin ʀ wrote.

By experience, you don’t need to hit all those parameters above so don’t worry about missing one or two of them for the perfect encoding. Not all programs come with such detailed encoding settings but try to hit it close.

Remember to check the aspect ratios (go with Square) and size of the video though (convert the video size if you have to). If they’re not correct you will get those dreaded black lines on the sides of your NND video.

Good luck uploading!