For this tutorial, you will use an image of a transparent object to apply the Mask to. This technique works better with dark backgrounds.
1 - In Photoshop, open the image you chose to work with.
2 - Go to the “Channels Panel” and check out every single “Channel” to choose the one with a better “Contrast”. Since a channel is a “Grayscale” image you can use it effectively to create a mask, because a mask also uses grayscale (pure white=100% opaque and pure black= 100% transparent) to store transparency information.
3 - In our case, the “Cyan Channel” was the one with the best contrast. The idea is to remove the black portion but to keep the glass, water, bubbles and the reflection below the glass.
4 - Drag the “Cyan Channel” Thumbnail onto the “Create New Channel” button at the bottom of the Panel to duplicate it.
5 - With the duplicate selected, go to Image > Apply Image and use “Multiply” as “Blending Mode” and a 50% “Opacity”.
6 - Repeat the “Apply Image” but this time use “Overlay” as “blending Mode. Keep the “Opacity” at 50%. Depending on your image you may need to alter the “Opacity” value and maybe to use the “Overlay” Mode a couple of times.
7 - Use the “Brush Tool” (B) with a Soft and Medium brush (0% “Hardness” and 55 px) to “Fix” some details in the duplicate. Choose Black to remove stuff and White to add.
8 - Now, use the “Dodge Tool” (O) with a 50% “Exposure, and focusing on the “Highlights” bring out the lighter portions.
9 - Do the same with the darker portions or to erase some unwanted white this time using the “Burn Tool” (O) with a 50% “Exposure, and focusing on the “Shadows”.
10 - Make a selection of your duplicate by holding the “Command/Control” key and clicking the Thumbnail or by clicking on the small button at the bottom of the “Channels Panel”. Head back to the “Layers Panel” and duplicate the “Background layer” and drag its Thumbnail onto the “Create New Layer” button at the bottom of the Panel.
11 - Create a “Mask” of the selection by clicking on the small button at the bottom of the “Layers Panel”.
12 - With the bottom Layer selected, create a “New Color Fill Layer” by clicking on the small Black and White icon at the bottom of the Panel and choose “Solid Color”. Choose a bright shade of Red to test the “Mask”.
13 - Select the “Mask” Thumbnail and using the “Brush Tool” (B) with Black as “Foreground” color, “erase” the portions that need it. You can use White to “bring out” some portions of the image too.
14 - Change the “Blending Mode” of the Layer to “Lighten” to get rid of the Grey.
15 - Make a selection of the “Mask” by holding the “Command/Control” key and clicking the Thumbnail.
16 - Create a “New Levels Adjustment Layer” by clicking on the small Black and White icon at the bottom of the Panel and choose “Levels”. Head to the “Properties Panel” and drag the “Midtones” slider almost all the way to the left placing it really close to “Blacks” one.
17 - You can use the “Clone Stamp Tool” (S) to fix any edges that may need it.
18 - Now, use the “Shift” key to select both the Top Layers (“Levels Adjustment” Layer and “Background Copy”) then, click on the small Folder icon at the bottom of the “Layers Panel” to “Group” them together.
19 - You can change the color on the “Color Fill Layer” to check out your masked image. Or you can add a “Background Image” and adapt your transparent image to that.