How do I specifically find handwriting advice videos on TikTok 斗音?
search for 臧老师教你写好字, it's an account of a primary school teacher, he posted lots of videos teaching 6~7 years old students to write Chinese characters.

How to find more this type videos? Search for the keywords 教写字, and you'll find more related videos and accounts (eg. The account 熊猫教写字-1年级 in the figure below is really good. Do remember to click the button 查看全部用户 to find more accounts and follow them)