I am probably not the best person to guide you here considering that I'm myself poor in Verbal. But here are some of things that I've tried that did help me to improve upon my scores.Do 1 thing at a time
: Do not stress yourself by trying to simultaneously do RC/CR/SC. Find out your weak areas, list them by priorities and tackle them 1 by 1.
Lets say you start with SC first (which is a weak area for most & also the most scoring). Get hold of a good book (Manhattan SC , i would suggest). Dont study from multiple sources. It will not do any good and you will end up confusing yourself with all the conflicting rules. Go through all the concepts chapter by chapter. Do all the exercises given behind the chapter & also the OG problems mentioned. Evaluate each question in detail (even the ones that you have got right). Take a notepad & write down all the mistakes that you have committed/ things that you have learned . After you complete the entire book (it should take you around 2-3 weeks at max.), redo all the questions you got wrong, see if you have learned from your past mistakes or are repeating the same. Again note down all your mistakes, and come back to it again after a few days. Focus on the basics first & things that you can do. Things that are difficult to understand, special cases/ high -fundoo rules....just leave it for now. This should help you to get a decent accuracy. Always remember, SC is beast that can be tamed easily and when you do, the rewards are high.
Then move on to CR (PowerScore bible) Repeat the same procedure as mentioned above. Do each chapter & the following questions. From OG, do 20 questions everyday, trying to understand the logic behind each question. Within a week, you can easily finish the book & all the OG questions. After this, I think the best place to improve your accuracy and understanding is through practice. And what better place than our best friend GMATclub. Our friends (god bless them!!) have posted the directory of questions topicwise. Read the relevant theory & practice questions from that particular topic. Start from the range of questions you are comfortable with, but challenging too.(600-700 range is a good place to start). Dont overdo. 20 questions per day is more than enough.
For RC, I cant say much, for that is my weakest area. Get a good book with good practice questions.(Aristole 99). Find the strategy that best works for you. I read the entire passage before going for questions. Do 3 passages everyday. Make it a habit. Do it for 2-3 weeks and see the change yourself. Cant say much on this.
You are already good in Quant, but maybe to score higher, you need to challenge yourself with harder questions. Get Gmatclub tests
. Do 1 daily & review all questions you get wrong. I normally copy paste all the questions i get wrong in a word file, to redo them after 3-4 weeks. That helps me to test whether i have understood the concepts.
I think, you can squeeze all of this in your daily routine( depending on no. of hours you work) and complete this within 2 month time period. No need to fret over extra material and non-official sources. If you need to do more practice, get the gmatprep questions doc & do some questions from them. That should suffice.
Finally, do it at your own pace & if you can get some tutor/friend/classes, more the better, but in no way a necessity.
I hope it helps. All the very best.
PS: I'm giving my gmat for the 1st time on 22nd May. Started studying from March onwards and was pretty much on the same boat. But I think i've improved and so can you.