This is a fantastic white boarding scenario, printing out and sticking up each part of the customer journey. Failing that, using presentation software can suffice. Just screenshooting and creating flow charts.
Where possible, overlay relevant data at each step, number of people each reaches, the click through rates, attention, drop off, conversion etc. This helps to understand how well each component of the journey works.
Many brands struggle with that, as they make it too complicated. This is an exercise, where simplest is best, to start with that baseline understanding. And then you can work from there.
Analytics plays a great sidecar role in customer journeys, providing that feedback loop of how well its working. And identifying areas that need improvement.