It a lot of the time depends on the case, as sometimes it is really easy to tell what is wrong, other times, you have to do a little poking around or researching your problem.
Oftentimes, there will be errors such as "UNHANDLED EXCEPTION DURING GAME FIBER TICK" and will directly tell you what the issue is (such as missing files, plugin issues, etc). Other times, all you will get is a line and all that it gives you is "Navigated away whilst loading" (or something similar) which is where, unless you can correlate that to a mod, you will probably need to do some researching if you are not someone that regularly reads logs.
Loaded plugins will also provide you with information about outdated plugins, but you will need to find the plugins when they loaded.
Regardless, there will likely be a few times that you need to Google something, which is fine because logs are not always direct when it comes to finding issues. All you might be able to do sometimes is temporarily remove files and reinstall them to find the issue.
There might also sometimes be more than one crash warning or reason, so it might not always be towards the bottom. Likewise, there will be some cases (very small amount) where you cannot do anything except for removing the mod. (Such as "vector" errors)
If you have other questions do let me know, can be a little confusing for first time readers, but eventually you should be able to do it confidently enough.