1. Open a 12"x12" empty canvas or pre-made paper 
(height doesn't matter, but width should definitely be 12") 

2. Select the drawing tool 

3. Go to tool options and select "Bezier Curve", with "antialias" 
and "create as vector" checked. 

4. Set your palette colors. Foreground color (top color) can be any color, 
Background color (bottom) is set to null (circle with a slash through it). 

5. Holding the mouse button down, draw a line across the full width of the canvas. 

6. Let up and click the mouse above the line you just drew about 1/3 of the way across. 

7. Click the mouse again below the line you drew about 2/3 of the way across. 

Now you have a curved line that you can stretch in any direction to customize exactly 
what you'd like it to look like. 

8. Customize your line, being sure to stretch it all the way across the paper. 

9. Go to "Layers", "Convert to Raster Layer". 

At this point you can either save your curved line for future use as it is or you can use 
your magic wand tool to select the area above or below the line, change to the background 
layer and "edit", "copy" that selected part of the background layer to paste as a new image 
to have a bigger template for future use or for use in your layout. 

Bonus Tip: 
After cutting out your curvy-edged paper, use your magic wand tool to select the area outside 
of the paper (the transparent, cut out part), then go to "Selections", "modify", "Feather", 
and set it at about 3, then hit the "okay" button. Now hit the "Delete" button on your keyboard 
a few times to soften the edge to avoid any jaggies! Always a good idea.