Resolved Child Products - Making Existing products to child

Hello,

Also i can't find any guide or tutorial about child products.

There is not just one type of child products. For example there are the Multi Variants, which is a type of child product as well.

I made my first import with fields "SKU" and "Parent_SKU" and indeed the products became parent and child.

That is the simple parent/child relationship.

How can i add into parent products the custom field "Generic child variant"

Like any other custom field, see the Product import with custom fields tutorial for more details on using custom fields.

and into custom field i want to set the value product_length and not the product_name

This you will need to explain further as I don't understand what you are trying to achieve here.

How can i remove the categories from the children products.

There is no such option in CSVI. The only way I see possible is to empty the table virtuemart_product_categories using PhpMyAdmin for example. After that do an import of your products with the category_path to assign the categories to the products you want to have a category.