There are actually a lot of different ways to measure your core web vitals. Some of them may give you slightly different results but I think the best way is to measure it in your browser. All of the browsers have the capability of giving you some scores back but you might want to start with Chrome.
If you go and open the Dev Tools under the web developer menu it'll bring up the panel which includes all kinds of web developer tools for you to interact with. So, you go to the page you want to get scores from and you load up Dev Tools and then you click on the Lighthouse tab. It's important that you aren't bringing resources up from the browser cache too, so there's an advanced setting that you're going to want to switch under the gear icon. Where you have settings for your Dev Tools you should find a setting that says 'Disable cache'.
This is important when you do these tests because you don't want to get a false report that's the best cached data because you're working with your own computer. To simulate the best real world experience that your users will get and which is also the information collected by Google in a separate database called CrUX or the Chrome User Experience Report. You can get access to that data because it's publicly available they update it once a month. That's another way to get your scores that you're collecting out in the field, the same ones that you'll be ranked against in the new upcoming algorithm.This question also has an archived answer.