I don't think it's possible to upload a YouTube video directly to DTube, so the process would be the same.
You first need the video on your computer. The next step is to upload it to DTube.
If you're looking for a way to download YT videos, I would suggest something like:
https://www.onlinevideoconverter.com/video-converter
That allows you to download YT videos in both audio and/or video formats.
I know how to download but looking for a way to upload from YT. Folks are trying to move to DTube from YT.
I see. So you're basically trying to find a solution to move YT videos from YouTube to DTube. Well, that's probably more or less impossible for now, as @dtube doesn't support multiple uploads at the same time.
I did request a feature to upload multiple video files, or videos in parts, but I think we'll have to wait for something like that. Until such a feature comes along, we'll have to upload videos one by one.
They might be able to come up with a cool solution to move an entire YT channel's videos to DTube instead, but I don't think that's likely. Manual uploads, one by one, is probably the only option for now at least.
As it stands right now unless they have the video on their home computer they can't upload to DTube.