How To Edit this Point Of Sale UI

I tried with this method [Point of Sales UI - #2 by rohit_w] but its not working how to customise this page?
Eg:just need to add some text in middle of a page

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For each items selected, I want to connect a corresponding Salesperson/Sales Partner ID. How to change the ChildTable/ItemLines. So that, at the end of the day, I can calculate the Sales Report by each salesperson. For Example, For each product sold, there is a corresponding Salesman.