Using Download Helper
Download download helper, a free firefox add-on, to help download videos from the web to edit educational and creative (not for profit) content. You have to use it in firefox. Download here: https://addons.mozilla.org/en-US/firefox/addon/video-downloadhelper/
You can also use other programs or methods to acquire your content if you prefer. Always try looking for "official" videos, or sources that you recognize (UBU Web, Internet Archive, NASA, libraries, the government, arts institutions, archives, etc). These organizations will often go to great pains to make sure the highest possible quality is available, and often for free. Take advantage of these archives, as they will often have the best quality versions available.
With digital files, you always want use the highest quality video file you can find. Quality is the most important thing, because while you can always degrade files, you can't upgrade them. (You can't make an SD file HD, but you can the other way around.) You want the largest possible dimensions, which will translate into a larger image and file size. Always get stereo audio if you can. All of the file sizes listed below will likely not be available for every file.
Using Download Helper:
First, open firefox and go to youtube or other, similar site. Then, identify the "download helper" icon to the right of the url bar (where you see/type in the website address). It is a red, yellow, and blue glob. It will be animated when there is a file you can download. Then, click on the little down triangle next to the glob. It will tell you what file options you have. You will hover over the one you want, and select download. Be sure to save it in your class folder so you know where it is.
First, try for a .mp4 or a .mov (which is apple specific) file. If possible, try to get something that is 4k, 1080p, or 720p. On youtube, your most common .mp4 files will be listed like this (in descending order of quality/file size):
[HQ 38] or 4k (4096 x 3072), stereo
[HQ 37] or 1080p (1920 x 1080), stereo
[HQ 22] or 720p (1280 x 720), stereo
[HQ 84] (960 x 720), stereo
[HQ 85] (720 x 540), stereo
[HQ 82] (480 x 360), stereo
[HQ 18] or Medium (360 x 270), stereo
[HQ 83] (360 x 240), stereo
you can also use FLV files:
[HQ 35] or 480p (854 x 480), stereo
[HQ 34] or 360p (640 x 360), stereo
[HQ 6] (360 x 270), mono
[HQ 5] (320 x 240), mono
Notice here the video size (in parentheses). Always get the highest quality you can. You can always use Adobe Media Encoder (included with your CC download) to convert files when you need to, but it is best to start with the highest quality. It is also important to note that in Premiere, you cannot use different file types in the same sequence. They must be the same, or else you will run into problems. We will talk more about what this means in the program.