The system was based on accurate measurements. But complete instructions were given.
Here is one dress and the pattern pages you'd need to make it. There were pages for every garment, you looked at an index and it told you what pages you needed to use.
For the Basque (one or two darts);
For the Turn Down collar;
The Sleeve (two to choose from);
And then choose between two Skirts;
Just consider the work this took, there were limited fabric choices, no high powered machines like we'd use today, and imagine washing these!