I am not a designer, so have created this tut with other people's handy work. Thanks to Gratefull4U kit, yellow Duval Street paper by Elizabeth Quintana and photo frame by by Viviane at http://viviane07.blogspot.com/
Step 1Open the background paper of your choice in it’s own layer. Create a new transparent layer, use the create new layer button on the Layer dialogue for short cut.
From the GIMP main menu, select white as the foreground colour. Select the Rectangle Tool next and click/drag your mouse to draw a narrow rectangle. Fill it with white Foreground colour.
From the Filter menu, select Distorts then Curve Bend
Click points along the straight line and drag points to represent the desired curve. Select the copy button, this will ensure that the curve is copied to the lower edge of the rectangle, then select OK.
Change to the Fuzzy Select Tool and select the white of the curved layer. From the GIMP menu select the 2nd red circled mode, and with your mouse select the space under the white curve and fill with white Foreground colour.
Duplicate the solid white curve layer in the Layer dialogue. Add new paper to the layer stack above the duplicate curve layer. Cut new paper from the shape of the duplicate curve layer and Merge Down layer.
To create the thin white border between the curve and background paper, select the white solid curve layer. From Layer Menu select Autocrop layer. From GIMP menu select the Alignment Tool, set Offset to -20, and select bottom right Distribute button.
This tutorial can be applied to curving text and other elements such as straight stitching too.