Using Mask Layers - Basic

Step 1 - Display Layers palette, open two photos
If it isn't already displayed, display the Layers palette. Now open two photos, ideally photos of the same size or scale, and one of which has an element that would look somewhat natural if placed in the second photo. For example, open two photos containing people, or one with people and one with a landscape.
Step 2 - Copy one photo to the clipboard
Decide which photo contains an element that you'd like to combine with the other, and make this first photo the active one. Now copy all of the photo (by pressing Ctrl + C, or choose Edit > Copy) or just the desired part of the photo (by using one of the selection tools -- you don't have to be precise) to the clipboard.
Step 3 - Paste into the second photo
Make the second photo active, and then press Ctrl + L, or choose Edit > Paste > Paste as New Layer. Notice in the Layers palette that this is now the active layer. If necessary, use the Move tool to position the layer as desired.
Step 4 - Create the Mask Layer
With this new raster layer active, choose Layers > New Mask Layer > Show All. As you can see in the Layers palette, this creates a Mask Layer grouped with the new raster layer. Notice that the Mask Layer is now the active layer.
Step 5 - Choose the Paint Brush tool
From the Tool toolbar, choose the Paint Brush tool.
Select this tool for me.
Step 6 - Set colors in the Materials palette
In the Materials palette, set the Foreground/Stroke style to the Color option, and then set the foreground color to pure Black. Likewise, set the Background/Fill style to the Color option, and then set the background color to pure White. (Masks work by hiding all pixels painted Black, and conversely showing pixels painted White.) Click this step's Script below to do this automatically.
Reset the colors for me.
Step 7 - Paint on unwanted image area
Place the Paint Brush cursor over pixels you want to hide, hold down the left mouse button (to paint with Black), and then paint with slow, careful strokes to hide these pixels, revealing the image below. (If necessary, adjust the brush's Size setting larger or smaller as needed.) Continue doing this until you've isolated the desired image element, making it appear as though it was part of the underlying photo. If you mistakenly remove part of the desired image element, simply paint over the affected area with the right mouse button (to paint with White), restoring pixels.
Notes and Tips:
Masks can be a fun and relatively quick way to create photo composites, and even provide an effects-like look to images. Refer to the Help system or the User Guide for more information on masks.
Try choosing Layers > Load/Save Mask > Load Mask from Disk, and then select one of the sample masks that come with Paint Shop Pro.
Be aware that you can adjust the brush size via the keyboard. Refer to the Help system or the User Guide for details.

Learn More:
Using Mask Layers - Intermediate
Erase an image background
Create depth of field