In order to improve SC I recommend you do the following:
1. Manhattan GMAT SC Guide - buy it and read it over and over. Ideally breaking up the material in different study settings. There are approximately 9 chapters. So do 1 in the morning 1 in the afternoon for a week. You need to practice alot to be exposed to many types of questions - all types of questions.
Learn and study the content, do the exercises in the guide.
2. After going through the manual two - three times you might have doubts, some gaps in your reasoning, nonetheless you will feel that you can answer SOME questions. This is good. You will develop confidence as you can answer some questions. For those which you can't use every single question as a learning session. Use every single wrong choice and identify the error. By applying your content knowledge you will be able to pinpoint what you know and what you don't. Do 10 problems in a setting. Take a 5 min break and look up the 40 wrong answer choices and identify why their wrong. You will learn a lot. This will help you close gaps in the SC theory and will also help you improve your application.
3. Now, go through the MG guide again and skim thru it in a pdf and make comments alongside what you STILL don't know. Write it down, study it - do whats best so you know it.
4. Practice more do 100,200, 300 problems. Repeat the ones you did earlier close the thinking gaps. Keep a detailed excel file with all the questions attempted and whether you got them wrong. This will help you differentiate what you have yet to learn and what you have learned. By now, your confidence will be much higher and your approach will start becoming more systematic.
5. Develop a full proof approach...
Ex:
Read sentence
Identify error
Look for options linearly to see whether they fix the error.
OR
Read sentence
Look at the options
Eliminate based on each error.
There's thousands of variants to getting the right answer - find the approach that works best for you and get to the answer.
I hope any of this helps... Its all a mix of learning the theory, applying it in the questions, and relearning and finding out whats missing. Then time factor comes in. I understand that you can improve RC and CR but if you don't improve your SC the whole Verbal section is impacted. You need to practice, practice, practice, review, review, review. No way out.
All the best and best of luck!
Jas