To add a safe custody item in the Legal Details window within a particular matter, you will first need to create a packet for the item. To see how to do this, refer to this article.
Once you have created the Packet, within the Legal details window for a particular matter, click on the Safe Custody tab. You can then click on the New Safe Custody button in the Top Toolbar. The Update Safe Custody window will open up. Refer to this article to see how to fill this in.
Once you have entered all the information, click on the Save button on the bottom right-hand side of the window to save the Safe Custody item.