Masking in Singular is done by adjusting the bounding box size of the parent group that the image lives in and activating the "Clip Children" feature.
1 - Turn on Bounding Boxes
Click the bounding box icon in the Preview Toolbar to toggle bounding boxes on and off. Having them on will help you see the area that you're masking.
Turn on Bounding Boxes
2 - Place Element Within a Group
Make sure the element you want to mask sits inside a group. To add a new group, click the "Add" button in the upper left, then select group.
3 - Clip Children
Within the parent group, go to the Effect tab in the Property Panel and check the Clip Children box. This will ensure elements within that group remain within the confines of the group's bounding box.
Click the "Clip Children" button
4 - Adjust the Size of the Bounding Box
Go to the Transform tab in the Property Panel and adjust the size of the group's bounding box to set the area you wish to mask to.
5 - Adjust the Element Size
Click on the element you wish to mask and adjust the size to your liking. You'll notice that the element is restricted to the area of the parent bounding box.
Mask an image by adjusting the size of the parent group's bounding box