I'm aware that the mathematics GRE subject test largely tests calculus, linear algebra and a bit of algebra at the freshman or sophomore level. These constitute the firsgt two categories on the exam, mentioned in the official booklet available here. The third type of questions fall in the Additional Topics category. This catgeory includes questions on:
Introductory real analysis: sequences and series of numbers and functions, continuity, differentiability, and integrability, and elementart topology of $\mathbb{R}$ and $\mathbb{R}^n$.
Discrete mathematics: logic, set theory, combinatorics, graph theory, algorithms; and
Other topics: general topology, geometry, complex variables, probability and statistics, and numerical analysis.
Like it or not, this ctageory constitutes 25% of the questions on the exam! I'm unsure of the best and effective way to go about preparing for this section. I suppose one has to do (extremely) well on this section of the test as well to score in the $85/90 +$ percentile range.
I'm well aware of the studying tips for the other portion of the exam; for one, pick up your calculus textbook and start practicing! I'd really appreciate if someone could outline a study plan to effectively prepare for the last portion of the exam, without assuming the student may have done all the topics (mentioned above) as coursework in college. It'd be great if you could mention some specific resources/books as well.