An open source cross referencing add-on for Google Docs that automatically numbers elements such as figures and updates and formats in-text references to them.
From the Google Add On Store or by clicking here.
First and foremost, by inserting labels and references into you document. A label is a URL made up of a #, a 5-letter label code such as figur, an _ and a unique name for that element. For example, a label for a figure that you want to call population would be #figur_population.
You add this to text -- like the word ‘figure’ below your figure -- as you would any other URL (by highlighting, then hitting Ctrl+K or ⌘+K and entering the url). You can now refer to this figure in the text any number of times using the URL #fig_population. References use a code which is the first three letters of the corresponding label code.
Once you have some labels and references, you can update the document by going to the Cross Reference sub-menu in the add-ons menu.
The wiki covers the use of Cross Reference in more depth, including customising labels and references. Please feel free to raise issues or ask questions!