The best way to prepare for the GMAT 2018 download free pdf sample papers from various online sources and study them along with the answer sheets.
GMAT 2018 sample papers
These GMAT 2018 sample papers are available all over the internet, and most of the reference books also have an entire section dedicated towards them. Take these tests, and analyze your results after each of the tests. See where you are making mistakes, keep a record of your score and make sure that your scores are improving steadily.
Figure out where you are making most of the mistakes, and look for the answer sheets for them. Practice those questions more and try to learn various tricks to solve the questions you are the weakest in. Look at the answers to the questions that you solved right, see if they have a better way to come to the solution than yours, if they have, practice that. After all, you have limited time to solve these questions.