Hi Sarah,
Here's one way you can add one hand from one photo to another:
Open both photos in Photoshop
Go to the photo with the hand you want to use
Make a selection of the hand using any of the selection tools--try using one of these: the Object Selection tool set to Lasso mode in the Options bar, the Lasso tool, or (if those seem to tricky) the Rectangle Marquee (a dashed rectangle icon). If you have any trouble finding any of these tools, you can use the Discover Panel to help locate them for you. Go to Edit > Search and enter the tool name in the search field.
Select the hand (you may want the selection to include additional pixels around the hand that you can remove later)
Copy the selected hand
Go to the other photo and paste the hand (Photoshop will put it in a new layer)
Remove any of the hand layer image using a layer mask (Layer > Layer Mask Reveal All)
Select the Brush tool and set the foreground color to black and the background color to white--at the bottom of the toolbar, click the white and black small squares icon (Default Foreground and Background Colors (D) will appear in the tooltip when you hover over it) to set the colors to default if the colors aren't already white and black. Then use the corner arrow icon (Switch Foreground and Background Colors (X) is the tooltip that will appear when you hover over it) to the right of the small white and black squares icon. When the foreground color is black in the toolbar and the layer mask (the thumbnail to the right of the layer thumbnail in the Layers panel) is selected, draw with the brush to remove pixels that you don't want to appear.
You may need to select the hand thumbnail (to the left of the layer mask thumbnail) and use Edit > Free Transform to rotate or adjust the hand. There are other more advanced tools for warping the hand if that's needed.
When you're done, File > Save your layered file and use File > Save a Copy to save the edited image in a flattened format to keep your layered version if you need to go back and make changes.
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