Hi there.
Sometimes channel URL looks like this:
https://www.youtube.com/user/nuggetnoggin/
and You need it to look like this:
https://www.youtube.com/channel/abcxyz123
How to figure out what the channel ID is? Quite simple.
I will show how its done in Firefox but I any decent browser will do it for you.
1) Grab (as in copy) this:
feeds/videos.xml?channel_id=
2) Go to the YouTube channel page.
3) Right click somewhere on the page and choose “View Page Source” from the right click menu.
4) Now press and hold CTRL and press F. Release both.
Paste the bit from above ^^^ into the search box.
Voila!
This user’s channel ID is UCm_Fbx0tAqAOB3cA4d1GnXw.
Cheers.
Andrzej
