The first step is to determine the number of inputs, outputs and check boxes required by your app. Adding an input later is unpleasant as you must start at the beginning, hopefully saving your calculations.
In setting up my app, I had to update the input and output boxes a couple of times. I had trouble (easily fixed) with scaling working with mm and m and other similar units. I found I did have to set the precision of numerical results. I added a bit of math to the minimum and maximum scales for the graph.
I burned about 4 hours in getting the app to work using sliders. The app works, but I want o add functionality.
The next day I worked on text input. And tidying up the app for general use. User help was written. I did not succeed with the text input. I did contact Prof. Abbott for a bit of help.