You want to start your image BIG, so open a new drawing at 8.5" x 11" in inches (612 x 792 pixels). This is double the size of the actual image that will be printed when making a card, which we want for better printing resolution.
Select Image/Guides/Centered Guidelines to find the center point of your image (you can drag out your own guidelines to center if you wish - 306 x -396y).
Create a new layer with a white background and move it UNDER the background layer, name it "soften". We will use this layer to soften the background layer by lowering its transparency over a solid white layer. You'll see why in the next few steps.
Here is where the tie-dye fun begins:
Select the gradient tool and input the following settings:
Drag a line down from the top center to about halfway down the drawing:
It will look like this:
Open the G'Mic plugin under the Filters menu. Select the Deformations tab, then choose Twirl. The settings are as follows :
(Do not close G'Mic after pressing "Apply")
Select the Water filter directly above Twirl, the settings are as follows:
Select "OK". Your result should be similar to the following:
Lower the transparency level on the background layer to around 50, to lighten it. (Note image below):
Create a new transparent layer above the background layer, name it Line Nova. Set your foreground color to white. Select Filters/Render/Line Nova from the menu and input the following settings and select "OK":
With the Line Nova layer highlighted, go to Filters/G'Mic once again, selecting the Water filter under the Deformations tab. Reduce the amplitude setting to 25, leave smoothness at 1.5. Select "OK". Lower the transparency level on the Line Nova layer to about 60, and you are DONE! The final result and layers set-up are below:
Coming Soon - GIMP Tutorial: Creating Psychedelic 60's Christmas Cards, that uses this tie-dye technique.