i think you are in "luck" since SC is arguably one of the easier sections to improve due to the rules and everything. Here's what i try to do:
1.read the entire sentence.
2. check for Subject-verb agreement - this gets tested a lot and makes 3/2 splits very easy to find. If the subject is "teenagers" then in your mind you should be actively searching for verbs that are plural like "are" instead of "is" etc. In a lot of sentences they try to mess you up with phrases that comes between subject and verb like "the teenagers, after playing football for 2 hours, are/is" Always try to ignore the "extra garbage" in between to find the plural or singular verbs
3. Idioms - this is done by memorizing a list and practicing. some you can probably figure out by ear but you should really memorize them.
4. Run-on sentences. Always check if the sentence is a run on and if it is you can fix it with FANBOYS(For,And,Not,But,Or,yet,so) or semicolons/periods. Remember when in doubt read the 2 phrases separately and see if they can be written off as individual sentences. If they can be then you will need to use FANBOYS or the other ways to not make it a run on sentence.
5. check subject after phrases - some of the question on GMAT might start the sentence with a phrase that is modifying a subject and you need to make sure it is the right subject. Something like "After weeks of revision, the book "Grammar A to Z", authored by Henry, is going to be published" vs. "After weeks of revision, Henry, the author of "Grammar A to Z", is going to publish it". Notice how the beginning phrase it is talking about revision - the 1st sentence i gave the subject is the book. The 2nd sentence it is Henry - obviously he isn't the thing being revised so the subject should be the book not the person. Again i am not the best person to give examples but i hope you got this one..
If you get really stuck you can also try to see what the major difference among all the answer choices are - is it is/are usage, amount/number, and punctuations etc.
If you like my answers please +1 kudos!