How to Import Categories, Sub-Categories, and Images

Hello,

Firstly, is there a Tutorial / Docs ?

1))
I am trying to do an import to begin with of only the Categories and Sub-Categories. Cannot work out how to do this.
a) how do I select only Categories
b) how do I assign a column to be a subcategory
c) what should the csv look like for this scenario

2))
I tried importing a set of products from a csv file that works with other plugins but I am getting the following error ;

WordPress database error: [Illegal mix of collations (utf8_general_ci,IMPLICIT) and (utf8mb4_unicode_520_ci,COERCIBLE) for operation ‘=’]

3))
how do I import the Feature image and Gallery images

thanks