I passed the CLD exam and would advise the following.
1) Read the requirements
2) Build the interface and document - this will give you documentation points plus increase you understanding of the requirements
3) Implement a simple producer consumer architecture with events documenting as you go - implement the simple stuff first (initialise GUI, stop loop) and document as you go. This will tick the boxes for style and give extra points for documentation.
4) Only now start to code main functionality. You should have a good understanding of the requirements by now and you should already be set up to implement a good style. Start at the beginning of the requirements and add one functional requirement a time testing and documenting as you go.
You do not need to finish all requirements to get a good mark - if the style is there and it is well documented then you should do well. If you don't get the style right then you will struggle with the functionality anyway.
Hope this helps
Planning up front is a very good idea. Look at what you are to accomplish and see what structure you need to do it. My CLD exam did not need producer-consumer and was very linear in what had to happen. Build modularly. Each funciton within its own block. Add a function/requirement - test it. Then move on. Always keep a functional/running version of the program. If you run out of time, submit the last running version. This will give you a lot higher points for functionality than if the VI is broken.
I certainly agree on document as you go. I tend to document first (this section does this... this section does this... etc.). It helps with the design.
The program I had to build had user interaction to start the process and that was it. No event structure at all.
Have fun coding.
The best way is to first extract small set of requirements, and create SubVIs, or LV2Gs. One those are done, you just need to put them inside a state machine and make a transition to the required state depending on the requirements. I used this method, and I was easily able to clear the CLD within the alloted time with a decent score 🙂