How to account for Stock taken for own use

What is the proper way to account for this, please? is it to write an invoice to yourself for the stock you used? Do you buy it from yourself at market value or the cost price or somewhere in between? For instance, If I was to sell t-shirts but wanted to just grab a t-shirt for my own use how would account properly for this please? Thanks in advance

Hi
If you are completing a paper SA103S, you would enter the cost of goods used in box 11 of SES1.  
If you are completing SA103F please show the costs in box 60 on page SEF3.  
The online box will be named "goods and services for your own use".  
You would enter the cost price of the goods used for personal use, not the selling price.