1. Hi. Should I do marking and making side notes during reading or after reading the passage?
Hi lathlee! You should absolutely be marking and notating your RC passages as you read them. Going back to complete notations after reading would take too long under the time pressure of the LSAT.
2. If Marking and making side notes during reading compromises to forget some details or delay some time to completely comprehend the passage?
Your marks and notations should act as roadsigns that remind you where specific concepts and information are located. It seems like your current method is to read the passage as fast as possible, and try to hold all the information in your head before you forget it. Instead, I encourage you to mark the passage in a way that will allow you to quickly re-locate important information. Remember, RC is like an open book test. You don't need to have everything stored in your head, rather you need to be able to quickly find things that you are asked about. Notate the passage with the mindset of "If I'm asked about this concept, I'll know the answer is right around here."
3. It happens with RC and LR time to time, I notice this happens to me all the time (But I do get some correct and some incorrect even with this state): what I do with when I have somewhat incomplete or somewhat confused understanding of Question stem or Question or Answer choices or combination of them ? (but I think I have a good grasp what the author is asking and important details.)
There are a couple of questions here. If you do not understand a LR question stem, you must take the time to understand it. There is no getting around this. Drill yourself on identifying question stem types until it becomes second nature. However, if it is an answer choice you don't fully understand, leave it as a contender and come back to it once you see how the other answer choices stack up. You may arrive at the correct answer via process of elimination, or there may be a subsequent answer choice that fits your prephrase perfectly. Some wrong answer choices are intentionally written in a way to tempt you to waste time deciphering them. Don't fall into that trap!
The situation is a little more lax when it comes to not fully understanding RC question types. Of course, you must still take the time to understand what is being asked of you, but because the great majority of RC questions are some variant of Must Be True, it's not quite as pressing to identify the exact question type. As for the answer choices, the contenders and losers method often works even better in RC. Strike answer choices that you know are wrong, and come back to answer choices that require more thought.
Most importantly, don't stress! If you're really hung up on a question, take a guess from the remaining contenders and come back to it if and when you have time. Every question is worth the same one point. Don't spend time stressing over a confusing question if it means you won't get to other questions that may be much easier for you.
Keep at it!