Hi! Getting a 680 is definitely realistic as you said you have three months of vacation to study.
1) Your question is a general question which many have posted in this forum. Maybe you might like to look at them as well.
2) Have a positive mindset about the exam. According to me, getting bogged down and feeling nervous will be the biggest contributor in your downfall. So let not a target score worry you too much. Don't get fixated to a particular score. Just give your best shot so that you remain motivated to bring out the best in you. Tell yourself every morning and every evening and whenever you feel down, "GMAT is testing some core skills and nothing advanced. Just push yourself with the mantra, "If I can't, then no one can"."!!
3) Give at least one mock every week until your exam day. Spend more time in analyzing your results. Improve wherever possible. Develop and improve your timing strategy. There are ways to improve on this front, like (1) more practice under exam conditions or (2) strategically skipping few "above your ability level" questions in few seconds or (3) setting up your scratch pad before the exam to keep a track of time etc or many other strategies. Refer to this for more details (
https://www.manhattanprep.com/gmat/blog ... nt-part-3/)
4) Official questions are like a gold mine. Solve them and learn from them. Analyze even those questions which you have got right. Analyze the wrong options why they were wrong. Maintain an
error log.
5) Develop a clear strategy on how to tackle each and every question type. This will help you in saving time. (For example - read this post:
https://gmatclub.com/forum/data-suffici ... ufficiency) - Similarly you will have to develop strategies for CR & other question types.
6) For relaxing yourself try doing some meditation on daily basis and eat very healthy food till exam. Some physical exercise on daily basis should also be helpful. Dont study too much just 1-2 days before the exam. Take control over your mind and be confident that you can do it!
I think I have shared few pointers which might help you. I hope some other users will chip in with some other pointers which I have not touched upon.
Wishing you all the very best!! I hope you get your dream score!!