Trying to cycle through a list of charcters

[pooky2483]

So, how do i shorten it down to 22 characters, as in a YT address;
https://www.youtube.com/channel/UC8d_Dl60fLdoK8cP-4bXx6E
And then cycle through
----------------------
---------------------_
---------------------a
---------------------b

etc...

-------------------dun
-------------------duo
-------------------dup