You can add accessories to each listing on the Write Up via the 'ADD' button.  The sum of the accessories is calculated in the 'Total' field in this section, and added to your Write Up.  To remove an Accessory click on the rubbish bin icon.

In 20.11.1 we added the ability to set up pre-defined accessory lists.  With this functionality it is possible to auto apply accessories as well as choose from a list you populated earlier.  You can read more HERE