To create a customer journey map for SAAS you need a big canvas. Whether it is a presentation, or an actual whiteboard. Then you can start by listing all the different ways people uncover your business and go through the purchasing process. Then try to connect the dots, as to the logical steps a user takes.
This can also be done with Sticky notes on a wall.
Where possible, quantifying each part of the customer journey, helps visualize how customers are moving through it.