Source for GRE content: PDF of Math Review
One way to use these is to visit each topic and, if you suspect you may already know it, do the practice problems first. If anything is difficult, KA provides links on the problem pages themselves to the related videos. Some topics are specific enough that I just linked you to the one video associated with it.
This could be improved, so please contact me with any suggestions!
This will just take practice, I think. This section is about viewing data -- as in a chart -- and answering questions about the information therein. A great way to practice for this would be to interpret informational graphics in the newspaper and/or figures in academic literature, for instance by reading an assertion in the article and checkign whether the data cited bears that out.