What you will need:
A basic understanding of how to use gimp's curves tool
A basic understanding of how to use gimp's layers and using gimp's selection tools.
An Understanding On Layer Masks (for the bubbles)
An Understanding On Layer Modes (Also for the bubbles)
Wacom Tablet (for editing the bubbles.)
My eBook on Layer Masks - This is an extensive, but short book that will help you get insanely good at layer masks. If you get good at layer masks, you'll get good at Gimp.
Start with this:
Part 1: How to Use GIMP to Get the Retro Colors
Most of the effect for this photo is simply from adjusting the curves of each channel (the red, green, blue, and alpha channels) like so:
|Note: To change which channel to adjust, select the channel from the channel drop-down. You can switch back and forth between channels. all of these curves adjustments should be done in ONE COMMAND, not a series of four commands.|
|This looks pretty cool, but we need to tone the contrast down a bit.|
|Not bad, now if only those bubbles didn't disappear in the process.|
The bubbles seem to have disappeared in this photo. We're going to use a combination of selections, and soft brushes to put some pop back into them.
Create a new layer, name it "bubbles"
Now we will select the bubbles. Using the path tool, trace around the outer edge of each bubble.
One all of the bubbles have been outlined, right-click on the path in the path menu (located in the same window as the layers,) and click "path to selection."
Set your foreground and background to a lime green and hot pink.
using a fairly large and soft brush, carefully brush in a few spaces of pink, and green in each bubble, like so:
|Now that we have added the color to the proper areas of the bubble, let's change some layer styles and adjust the opacity to make the bubbles look realistic.|
Duplicate the layer. Name the duplicated layer "bubblebrighten"
Set the duplicated layer mode to addition.
Adjust the opacity of both the "bubble" and "bubblebrighten" layer until you end up with something you're happy with. My opacity settings are set to 23 and 40 respectively, and they look like this:
|Now that our bubbles are a little more visible, let's dramatize the photo a bit.|
Create a new layer, name it "dramatize" set the layer mode to Overlay, and fill the layer with black.
Add a layer mask to the dramatize layer.
Using the blend tool, set the gradient mode to radial, and use a gradient that goes from black to white.
Create a gradient that goes from the center of the photo outward. Adjust the opacity of the layer to something you're happy with. Here is what I ended up with: