Select custom shape tool to create shape for fragment
Set to shape layer or path (top bar of Photoshop / control bar)
Go to the middle of the top bar (custom shapes options)
Select either 'add to shape area' (or as it is now known as combine shapes) or subtract from shape area option (depending on the design)
Select the subtract option to remove existing area from the shape (either use the same shape or perhaps circle or square)
How to use fragments or parts of custom shapes to create new path designs in Photoshop tutorial. You can use the full vector or you can remove the rest of the Photoshop custom shape and use only a fragment / part of the Photoshop shape. This can be done via the direct selection tool or using the set operation intersect / exclude / subtract tools. Fragments can be then combined in millions of ways for use as new designs in Photoshop CC CS6 CS5 etc. This tutorial shows you a number of ways of taking fragments or parts of a custom shapes and using the artworks in your work
Using fragments can be a great source for new PS vectors. It can also be saved as a new shape via the edit menu define custom shapes command.
Another approach to the above is to select two shapes and then use the layer menu and combine shapes command and subtract command
You can use custom shapes such as the sets on the site such as 13,000 shapes collection and snowflake custom shapes for Photoshop and elements and wonderful advertising themed star flash custom shapes and flourish designs such as scroll custom shapes
The Photoshop custom shapes on the left can be used as is (it is part of the shapes collection) but a fragment of the image can be used as well
The custom shape on the right was created by using the direct selection tool to select some parts of the earlier design and then delete key was pressed removing part of the design.
This was repeated with other points in the path. You can then define the shape as new Photoshop shape fragment.
The custom shape on the left. By using them with the pathfinder (combine, subtract etc) or set functions, you can remove parts of the path leaving a part which in turn can be used to create new vector and defined as such as via the edit menu.
Basic artworks can be used to fragment the artwork but you can also use the paths themselves to create ever more complex artworks. You can see the result on the right