The most important point of advice I would give someone who is new to dioramas and wants to make a great looking one is to layer. One of the most popular types of diorama when it comes to gunpla is of a grassy forest or rocky type area and most of the time the grass and the foliage looks flat and unrealistic. The problem is that just a single coat of flock is added to a plain base. I have seen videos of professional modelers do just that and make amazing pieces of work but unless you are a pro the results will just look bad. What I suggest is to first detail that base. Add a glue rock/dirt/sand mixture, paint it up, add pastels or pigments, highlight the lights and shadows, basically make it look as good as it would be if no grass was ever going to be on it. That way when you sift the foliage on there will have depth no matter what. It takes some extra work and might seem like it’s pointless but if you are inexperienced in dioramas it will result in better looking work.
Why I bring this up is because I am finished with the Char’s Counter Attack base and I added a extra coat of pastels (this time without mixing it with thinner). A light brown and blue mixture to cool down the highlights while lessening the earthy reddish tone.