For 99% of people taking the SAT, you're going to end up having to guess on at least some questions. While we're going to do our best to reduce the number of questions you have to guess on, we'll want to do everything we can to maximize the points you receive from the questions you do guess on.
There are two main scenarios where you'll guess on the SAT. One is when you're running out of time, and you just need to enter answers for the remaining questions. The other is when you've read and attempted to analyze a question, at least to some extent, but you aren't sure about the right answer. Our approach to guessing will be different for these two scenarios.
If you've had a chance to read the question and the answer choices, then always try to eliminate any answer choices that you either know are incorrect, or seem unlikely to be correct. The Bluebook testing app has a handy feature which allows you to cross out answers you know are incorrect. You should use this liberally, as it will help you to focus on the remaining candidates and not be distracted by the incorrect ones. And unlike crossing out an answer with a pencil, if you realize you misinterpreted something and that answer is back in the mix, you can easily uncross it. Once you've eliminated the incorrect answers, make your best educated guess among the remaining ones. How you identify incorrect answers will differ between the reading and writing sections and the math sections.
On reading and writing, you should normally be able to spot one or two that are likely incorrect. We'll get into more detail later on ways to spot incorrect answers, and how the College Board dresses up the incorrect answers to make them look more tempting. But even without this knowledge, you can probably eliminate one or two. Among the remaining choices, try to guess the one that feels the safest, that you can't find an argument to refute it in the passage. It may be a more vague paraphrasing of what the passage says, but the correct answer should be one that you can't find a diret fault with.
On math, it can sometimes be tougher to spot obvious incorrect answers, but on some questions it's definitely doable. For example, you might know that the right answer has to be positive, so you can rule out the negative answer choices. Or you might know there has to be an 8x because the slope is 8, so you can rule out the choices that don't have an 8x. But on some math questions, it can be difficult to spot the obvious incorrect answers. One thing to keep in mind is that they are normally going to have incorrect answer choices that are close to the correct answer. So if, for example, the answer choices are 3, 347, 428, and 452, you can guess that it's probably not going to be 3, because it's so different from all the other choices.
Anytime you guess on a question, you should flag it. Our brains have a way of continuing to work on problems even when we're no longer actively thinking about it. You may have an epiphany later while you're in the middle of a question, or you might have time at the end and be able to come back to it with fresh eyes and spot the correct answer.
If you're running out of time, you want to make sure you at least enter an answer for every question, so you'll want to quickly enter an answer for every question that remains unanswered. I think the 5 minute mark is a good time to do this. Don't break your flow if you're in the middle of a question, but if you're in between questions with under 5 minutes left, then take a few seconds to enter answers for all the remaining questions. When you do this, don't just pick a letter like "C", and enter it for all the remaining questions. Instead, try to make each guess as random as possible (more on this below).
On the old SAT, it used to be a pretty good strategy to just pick a letter, like "C", and whenever you have to blindly guess (like when you're running out of time), you'd just enter "C" for every guess. This is no longer a good idea. Part of the digital SAT's scoring algorithm tries to detect if it seems like you're guessing. If it sees that you entered "D" for 8 questions in a row, it will think you were probably just guessing. Even if the correct answer to some of those was "D", it won't reward you as much for the correct answer as it would have if it didn't think you were guessing. So, when you are blindly guessing, try to make each guess as random as possible. Something more like "BADDACBA" rather than "CCCCCCCC" or "ABCDABCD".