Once you're in Workday, the first thing to do is to define the type of calc field that you'd like to build:
On the next page, you're brought into a screen that functions in the same manner as the 'normal' reporting page. So you can choose your fields, as well as add in the more intense logic if you're building something more complicated. In my example, I just want to concatenate two fields together. I'm using email+mobile just as a sample, so that I get a mix of letters and numbers:
Next after you save your new calculated field, you can use it in your reporting. You need to choose the same Object (in our case Worker), to be able to find the field. Then, your new calculated field will exist like any other field on Worker.