Open a new document with a photo as seen below

Click on the "channels" tab

Left click on the color that has the most contract, in this case "Red"

Hold CTRL and left click on the "Red Channel", this will make just the flame become selected.
Once the flame is selected, left click on the "RGB Channel" which will bring the color back but the flame will be selected

Press CTRL+J, which will create a new layer but will only include the flame.
In order to see this, you will have to click on the "Layers" tab.

Delete the original layer (stock photo) and this is what you'll have