I would mark the location of every game element on the field with tape. Since the field specs are in the manual, you can mark the locations accurately.
Then, code the robot like the game elements are there. Have it drive over the marks that the discs would on, and when it goes to fire, make sure it’s positioned in a place that will mathematically get the disc into the goal.
Make it work perfectly in theory.
Above all, make your code easily adjustable. That way, when you get to competition and your programming skills isn’t perfect(It won’t be), you can easily make adjustments to make it work well.
Hope that helps!