Declaring commitment to Crocker's rules means that other people can optimize their messages to you for information, disregarding our cultures normal rules against potentially causing offense. The reason why one might commit to said rules is to maximize growth - in order to change yourself, one must first acknowledge that you're currently insufficient, which is made easier if someone is allowed to tell you so.
However, being able to abstractly reason that accepting blunt criticism is good does not allow you to accept criticism. When someone directly critiques you, it hurts. For most people, blunt criticism is net negative because the emotional state it induces is not conducive to change. For some people, normal criticism is also net negative for the same reasons.
Deflinching is a technique that attempts to create space to listen to criticism and Lines of Retreat is a technique to create the mindset to accept it.
At its core, Deflinching is just a TAP. It goes like this:
The heart of the technique lies in "making space". This is akin to a micro-focusing move, where the goal is to determine what part of you got triggered/offended/bristly.
In general, there are good reasons for you to get offended. If someone I respect tells me I commited a social faux pas, the internal wince I experience is real. In my case, it's because I don't like being told that I hurt someone because I don't like hurting people. The temptation is to respond "no, I didn't do that" because I obviously didn't intend to hurt them. If there's something your instinctive reaction is trying to defend, you want to surface that thing. You don't want to squish it down and let it eek out the sides, but you don't want to let it dominate either.
The generalized temptation is to direct the wince-type-feeling back at the person who gave you the criticism. This is bad for a few reasons, but one of the main reasons is that this negatively reinforces people giving you criticism, which means you stop finding out about possible improvements you can make. The point of "making space" is to figure out what the wince-type-feeling is actually directed at. In many cases, it's directed at the world state in which the criticism is true - in which case, don't kill the messenger. In other cases, it can be directed at the form of the message, as opposed to the contents.
In practice, the easiest way to "make space" is to pause for a few seconds ("easiest" is relative; on an absolute scale, making space for criticism is hard ). There is a classic anger management technique that is to count to 10 whenever you get angry. The rationalist equivalent might be to recite the Litany of Gendlin(or part of it). You could think of it as you already having a TAP that goes "get criticised -> react" that you want to replace with "get criticised -> pause".
This is another TAP. It goes like this:
If the criticism is true and you don't care, then you can just let it go. Sometimes people tell me that my shirt doesn't look very good. If it's not a shirt that I think looks good either, then I don't really care if they don't think it looks good - after all, they agree with me! There's a failure mode where you just pretend you "don't care" about a lot of criticism - if you've made space properly, this failure mode shouldn't happen, but it is still worth it to keep it in mind.
Most of the time, however, the reason that you got triggered/defensive is because you do care. If you have a particular sort of brain, realizing you care whether or not the criticism is true makes you curious to figure that out, which means you're now in an eagerly-listening-to-criticism mode, and you've succeeded. For most of us, however, the realization that you care is not enough.
The line of retreat is visualizing a world in which the criticism is true. What if I actually deeply offended my friend with a bad joke? What are the consequences? I would try and pass an ideological turing test for why they were offended and apologize, explaining a strategy to not commit anything in the general class of offenses that that specific offense represented. This would be hard and take time, but overall probably not cause lasting damage to any of my relationships. It's bad, but not that bad. Making sure the criticism is not true isn't my final stand; I can always retreat to reality.
The point is to put yourself in a mental state where you are able to accept that the criticism might be true. This is a prerequisite to being able to learn/grow from accepting criticism.
The final step is to figure out whether or not the criticism is true. I have no advice for this step, except that it is probably going to be easier than the above.
Find a friend and ask them to tell you small and non-central flaws about yourself. Notice the feeling of triggeredness/defensiveness/bristlyness. Imagine a world in which you actually have those flaws and ask yourself how bad it is.
If desired, repeat with flaws that escalate in size (be careful you're actually ready for this, and not just eager to show you're a Real Rationalist).