Summarize the following:
The GIMP app icon resembles a dog-like face holding a paint brush in its mouth. GIMP may take a few minutes to open if you haven't recently opened it. Click File in the top-left corner of the window (Windows) or the screen (Mac), then click Open... in the drop-down menu, select an image to open, and click Open.  You can repeat this process for multiple photos if you want to layer the transparent image over a non-transparent one. If you open a second image in a second window, select it, press Ctrl+C (or ⌘ Command+C on Mac), select the first image, and press Ctrl+V (or ⌘ Command+V. This tab is at the top of the window (or the screen if you're on a Mac). A drop-down menu will appear. It's near the top of the Windows drop-down menu. Selecting it prompts a pop-out menu. You'll find it near the top of the Dockable Dialogs pop-out menu. A small "Layers" window will open. Click the name of the image that you want to edit. It should be in the middle of the "Layers" window. Click and drag the "Opacity" slider at the top of the window to the left, or click the downward-facing arrow next to it to lower the opacity.  Make sure that you're monitoring your image while lowering the opacity. Lowering the opacity from 100 to 50 should be enough to make your image transparent. Click the X in the top-right (Windows) or top-left (Mac) corner of the "Layers" window. It's at the top of the GIMP interface. A drop-down menu will appear. You'll find this option near the bottom of the Image drop-down menu. Doing so "flattens" the image, thus preserving its transparency. The default export settings will save your project as a JPG, but you'll need to change this in order to maintain your image's transparent background:  Click File. Click Export As.... Find the file's name at the top of the export window. Change the "jpg" part of the file's name to "png" (for example, "bear.jpg" would become "bear.png"). Click Export at the bottom of the window, then click Export when prompted.

Summary:
Open GIMP. Open an image in GIMP. Click Windows. Select Dockable Dialogs. Click Layers. Select your image. Lower the "Opacity" value. Exit the "Layers" window. Click Image. Click Flatten Image. Export your project as a PNG file.