How to create a colour by number in GIMP

Creating the outlines:

1. Open a picture. It should be large with either relatively high contrast or a variety of colours. You can increase the contrast in the program beforehand (or make any other changes you see fit). 

2. Go to Image > Mode > Indexed > Use custom palette and click the icon to choose a palette of the colours you will use to colour it (basically your pencil crayon colours). You can download and import the palette I used, which uses 11 pencil crayon colours which are blended together to create a total of 78 shades. Set Dithering to None. Click Ok. 
My palette:
https://drive.google.com/file/d/1-nPDJh_MROqyunjqRDsmFyDOHs5dJJm9/view?usp=sharing 

3. Go to Filters > Artistic > Waterpixels. The image generated by this filter is what your colour by number will look like when done. To change the amount of detail, change the Superpixels size slider. You want to make sure that the image is not too detailed or you will not be able to add numbers. If your image is large, the window might say that it's not responding; just let it sit for a second. When done, click Ok.

4. Go to Image > Mode and select RGB. Duplicate the layer.

5. With the top layer selected, go to Filters > Edge-Detect and select Sobel. Click Ok.

6. Go to Colours and select Colour to Alpha (at the bottom). Select black as the colour and set both Transparency threshold and Opacity threshold to 0. Click Ok. 

7. On the top layer, lock the alpha channel (click the checkered icon in the layers toolbox). Set your brush colour to your desired line colour and using the paintbrush or pencil tool with a large brush size, colour in the entire layer. You should now have the outlines of your colouring page. 

Adding the numbers:

8. Download, unzip, and import the number patterns from here:
https://drive.google.com/file/d/1thMbv5z7GVktE7eSGjkOzxiu7jMEWpNJ/view?usp=sharing
I'll leave you to Google how to import patterns yourself because it will probably differ based on operating system and version of GIMP. Same goes for importing the palette.

9. Create a new layer between the outline layer and the colour layer for the numbers. Also make sure you have the palette editor window open somewhere (if not, go to Windows > Dockable Dialogs > Palettes and from there double click on the correct palette).

10. With the colour layer as the active layer, go to Select > By Color and click on a section of colour. Then use the eyedropper to find out what number that colour is (the colour will be highlighted in the palette editor and the name will be at the bottom of the editor). 

11. On the number layer, use the paint bucket tool to fill in the selection with fill type set to pattern and the pattern set to the number of the colour you've selected (so if your selection was colour 4, use pattern 4). 

12. Deselect everything (Select > None). Repeat steps 10 and 11 for each colour in the picture. Your colour by number is now complete.