Open the image in Photoshop
Unlock the background layer if needed
Select the subject using Select > Subject
Refine the selection using Select and Mask
Adjust edges, feather, and contrast as needed
Output the selection to a layer mask
Use the Brush tool on the mask to clean up remaining background areas
Use the Eraser tool only if necessary
Save the file as PSD to keep the mask
Export the image as PNG for a transparent background