Pretty much every repair I've posted uses just a few techniques.  Once you can use a few basic functions in GIMP, Inkscape, and OpenSCAD, you can print a repair or replacement for many things.

I start a repair by sketching things on paper, trying to identify the most important features.  I take measurements with my calipers and annotate the sketch.  This is usually enough detail to create a replica of something "blocky" like a bracket or simple lid.

Sometimes the thing I need to replace doesn't have just a few straight lines that can be easily replicated or measured (as with the plastic play structure horn cap or bud vase).  When that happens, I use reference photographs - sometimes with a common object for scale (such as the quarter used in the fairy wing replacement).

The photographs are best taken with the object against a contrasting background (either very dark or very light).

GIMP isn't always necessary, since Inkscape has a decent way to automatically trace images and adjust contrast.  However, I find that by manipulating the image in GIMP first, I tend to get better results.

Cropping the photograph, increasing the brightness and contrast, and even simply erasing parts of the photo can be used to simplify the subject of the photograph.  Usually I'm looking to turn the photograph of the object into a simple dark shape against a white background.  The closer I can get something to this, the easier it is to trace in Inkscape.

First import, copy and paste, or drag and drop your image into Inkscape.

From here, mMy go-to menu item in Inkscape is "Path -> Trace Bitmap".  This function allows you to have Inkscape trace the outlines of a photograph and create an SVG layer over the image.  Select and delete the image underneath, leaving you with just the SVG layer.

The next step is to turn this SVG trace into a series of smaller steps.  Select your layer and then go to "Extensions -> Modify Path -> Flatten Beziers..."  The "Flatness" option default is 0.2, which should be good enough for pretty much any project.

To manipulate the object further, we'll need to convert the SVG information into a DXF file for importing into OpenSCAD.  "File -> Save As -> Save as type -> "Desktop Cutting Plotter (AutoCAD DXF R14)" and save with your filename of choice and extension of "*.dxf"

Rather than creating an OpenSCAD tutorial, I'll just cover the most basic functions to get you from a DXF file to an STL.  Here are some examples directly from an excellent OpenSCAD wiki.

Linear Extrude

Example of linear extrusion of a 2D object imported from a DXF file.

linear_extrude(height = fanwidth, center = true, convexity = 10)   import (file = "example009.dxf", layer = "fan_top");

Rotate Extrude

Example of rotational extrusion of a 2D object imported from a DXF file.

rotate_extrude(convexity = 10)   import (file = "example009.dxf", layer = "fan_side", origin = fan_side_center);