Where can I find online writing skills practice tutorials?

Look it up in the internet.  TESTDEN is one of them.

Here you go............

http://www.niu.edu/writingtutorial/

https://www.udemy.com/writing-with-flair-how-to-become-an-exceptional-writer/?matchtype=b&couponCode=bd723351-b751-4e05-b83e-d8ab3efe4d48&utm_source=adwords&gclid=CO3gvuTxz8QCFQX3wgodQo8Ahw&utm_campaign=Writing-CLP-Search-SN&utm_term=%2Bwriting+%2Btutorial&utm_content=Course-2&utm_medium=udemyads