For the written case practice I'd recommend you to look at regular case interview samples (there is a ton here on PL) and then work through them on your own. Look at the prompt and core question you are trying to solve, then look at the data provided and work on your recommendation.
Follow these rules:
#1 Already have a plan when you go in for the written case
Since time is usually limited, you should have a plan on how long you want to spend on each task of the assignment beforehand. For practice, use 20 minutes for the analysis and 5 minutes for the recommendation communication. For the real case adapt accordingly based on the time budget provided.
#2 Focus – quickly separate crucial information from the noise
Written cases usually present you with an information overload that you need to sort out
#3 Graphs and charts – interpret and distill key insights from graphs and charts
Written cases bombard you with charts, graphs, tables, and other visual depictions of data that you should use to test your hypotheses. Learn how to quickly read and interpret them
#4 Math – quickly draft equations and conduct pen-and-paper math
Get into the habit of quickly setting up and simplifying calculations
#5 Storyline – draft a compelling storyline and tell it with visually appealing outputs
Create a top-down storyline of your recommendations. State your primary recommendation, then use supporting arguments to strengthen your position
#6 Presentation and defense – communicate and defend your recommendation top-down
If you have to present your findings at the end of the case, follow the top-down approach of your slide deck. Be confident and engaging when going through your recommendation and supporting arguments.
I have written in great detail about written case interviews in this free article here (including links to free prep cases from b-schools): https://strategycase.com/how-to-crack-written-case-interviews
All the best!