How to combine shapes to create a custom shape
There are two ways to create a custom shape:
- Make one shape from some existing shapes,
- Modify an existing shape.
To make a shape from some existing shapes, do the following:
1. Add shapes to the slide, for example:

When you select all shapes, PowerPoint shows the Drawing Tools toolbar:

2. Under Drawing Tools, on the Format tab, in the Insert Shapes group, when you choose the Merge Shapes dropdown list, you will find the following operations:

- Union shapes:
- Combines all overlapping and non-overlapping areas of shapes
- Subtracts nothing
- Retains formatting of the first selected shape

- Combine shapes:
- Combines non-overlapping areas of shapes
- Subtracts overlapping areas of shapes
- If selected shapes do not overlap, acts like a group with no option to ungroup
- Retains formatting of the first selected shape

- Fragment shapes:
- Creates new shapes from the overlapping area of shapes
- Creates new shapes from in-between empty areas
- Retains as shapes any areas that do not overlap
- Subtracts nothing
- Retains formatting of the first selected shape

- Intersect shapes:
- Creates shape from the overlapping area from all shapes
- If any shapes do not overlap, nothing will be created
- Removes non-overlapping areas of shapes
- Retains formatting of the first selected shape

- Subtract shapes:
- Retains the first selected shape
- Subtracts overlapping areas of other shapes from the first selected shape
- Removes all non-overlapping areas
- Retains formatting of the first selected shape

See also this tip in French: Comment combiner des formes pour créer une forme personnalisée.