Basically you have to link a specific expression to an Excel spreadsheet's cell. You can get started by adding an expression and clicking "Extended text entry" next to Formula field. Choose "Insert Function".
Here are all the functions you can use in NX expressions. Select category called "spreadsheet" and you get the functions to work with Excel spreadsheet. Simplest function is "ug_excel_read" where you define the spreadsheet and specific cell in that spreadsheet to be read to NX. "ug_cell_read" is similar but it can read text too which is handy in creating complex parametric models. Remember to give name to the expression you created.
Then you can use this expression in sketch for example to call a value from Excel cell.
This method is good for example for sketching and updating curves that have accurate dimensions given.
Excel is widely used for many types of automation in NX. You may need to define your design intention clearly. Where do you want to automate? Standard Assembly componets, similar parts with difference in dimensions (family parts), Just modeling a standalone component and making it as template?