A mini-review is simply the careful summary of one specific topic, supporting arguments and major points. It should be less than a 5 pages if using provided template.
General steps are the same as writing a full-length review article, including reading articles, taking notes, brainstorming and creative writing. But the mini-review may only focus on one specific topic or discuss one question. Example question could be like whether an algae pond should be covered; or discuss the progress of one specific catalyst.
Why write a mini-review:
1) You want to figure out the answer to a specific question/topic, and covering every corner is not necessary.
2) Often a research article doesn’t allow a review section over 2 pages, so you may publish it as a standalone mini-review article.
Pautasso M (2013) Ten Simple Rules for Writing a Literature Review. PLoS Comput Biol 9(7): e1003149. doi:10.1371/journal.pcbi.1003149