To begin this tutorial let me point out that you'll need this background image to startout with. You can make your own if you'd like, but this will just make the tutorial be a bit faster.


Import the background image into Gimp as a new document/project. Then go to Colors/Colorize and set the Hue to 117, Saturation to 46, and the Lightness to -76.

This will just make our background image have a green tint to it.

Make a new layer (name it "ball base"), and grab your Ellipse Select Tool (E) and left click and drag. While dragging press SHIFT+CTRL so it'll be a perfect circle, and expand out where you've clicked and make it a pretty big circle to fill the canvas however not to big, cause we still need to create a shadow below it later on in the tutorial. Once you feel the selection is good enough for the base of the ball fill it in with the color #ffab33.

Right click your "ball base" layer and select Alpha To Selection. Then make a new layer (name it "main ball shadow") and go to Select/Shrink and shrink your selection by 8px.

Grab your Blend/Gradient Tool (L) (Press "D" to revert back to default colors) and set the Gradient Type to FG to Transparent. Now make a liner gradient going up kind of like what I did here.


Now deselect the selection by going to Select/None (SHIFT+CTRL+A) and then go to Filters/Blur/Gaussian Blur and give it a 60px blur. Right click your "ball base" layer and select Alpha To Selection and then go to Select/Invert (CTRL+I) select your "main ball shadow" layer again, and press delete (or go to Edit/Clear) and this will get rid of the blur that exceeds past the ball. Then bring the opacity down to about 80%.

Make another new layer (name it "main ball gloss"). Right click on your "ball base" layer, and select Alpha To Selection then grab your Ellipse Select Tool (E) Press SHIFT+CTRL and then click and drag until you get a selection like this.


Now grab your Blend/Gradient Tool (L) and make a white linear gradient from the bottom of it to the very top. Then go to Select/None (SHIFT+CTRL+A) to deselect your selection. Go to Filters/Blur/Gaussian Blur and give this a 30px blur. Then right click on your "ball base" layer again and select Alpha To Selection and then go to Select/Invert (CTRL+I) select your "main ball gloss" layer again, and press delete (or go to Edit/Clear) and this will get rid of the blur that exceeds past the ball as done before. Additionally set the opacity to around 70%.

Make a new layer (name it "light gloss") and then grab your Blend/Gradient Tool (L) once again, and set the Shape from Linear to Radial, and give it a radial gradient to the top part of the ball like I've done here. (It's kind of hard to see so the image's background was darkened so you can see where I applied the radial gradient, Additionally I also set the opacity down to 70%)


Make another new layer (name it "base shadow" and make sure this layer is below the "ball base" layer). Grab your Ellipse Select Tool (E), and make a selection as seen below, and fill that selection with black.


Now go to Select/None (SHIFT+CTRL+A) and then Filters/Blur/Gaussian Blur and give it a 30px blur. Duplicate that layer by going to Layer/Duplicate Layer (SHIFT+CTRL+D) and then go back to Filters/Blur/Gaussian Blur and give the duplicated layer a 60px blur. Merge the layer down by going to Layer/Merge Down. Duplicate once more, set the opacity to 50%, and merge the duplicated layer down.

So far your project should look something like this.


Now select your "main ball shadow" layer and make a new layer above that (name this new layer "number"). Grab your Ellipse Select Tool (E) once again left click and drag in the center of where your top gloss part started and whole dragging hold SHIFT+CTRL like we did before, and fill the selection with black.


Grab your Blend Tool (L) make a radial gradient of white go out like so. (Ignore the new layer I created)


Now grab your Text Tool (T) choose Sans Bold for the font. (my size is 41, and I made the spacing -1.0) Grab your Move Tool (M) and move the text so it's in the center of the circle we created.

Set the text's layer mode to Grain merge, duplicate the text layer and set it's layers opacity to around 60%, and merge the first text layer into the number layer, and the duplicated text layer into the number layer. Now go to Layer/Autocrop Layer and use your Perspective Tool (SHIFT+P) to make it look like it's on the left side of the ball. Like so...


Now make a new layer (have this new layer displayed on the very top, and name it "top gloss") Now make an elliptical selection like the one seen below, and fill it in with white.


Now make another selection (like seen below) and delete/clear to get a look we want for the top gloss.


Now go to Select/None (SHIFT+CTRL+A) to deselect the selection, and go to Filters/Blur/Gaussian Blur and give it a 16px blur. (Don't forget to remove the blur that exceeds past the ball like we did in the beginning.)

This is what your image should look like so far...


We're now done, repeat the process for as many pool balls you want, and here's our final result.

After reading your tutorial, I tried to find a quick "sure-fire" way to make the numbers on the pool ball conform to the shape of the ball. I discovered a little trick, but it requires using G'MIC. It's pretty crafty though! Let me know if you're interested.