Using Youtube-DL is very simple. The app's main screen looks just like YouTube app, with the addition of a small bar on the upper part of the screen. Navigate through the app as you normally would, and when you find a video you'd like to download, just tap the button on the upper bar to do so. It's that easy!
Another positive point of YouTube-DL is it allows you download content in a huge variety of formats, more than the majority of similar apps. Download videos in MP4, WEBM, 3GPP, M4A, WEBM Audio and MP3. On top of that, depending on the format you're using, you can select from different levels of quality. This way, you can download content in the exact format and quality you need.
Lastly, you can access and manage all your downloaded content directly from YouTube-DL. Give this useful app a try!