What is the best way to propose one remote collaboration of this kind? This question is a followup to this previous question of mine. Let me quickly summarize. I'm a Physics PhD student interested in one particular subfield which appears to be very active and promising. I have studied some key papers about the subject and I do understand them, but I really don't know how to start doing research on the topic. I asked if I could remotely propose some kind of remote collaboration with some postdoc working in the subject to learn and gain experience and by the answer I get that it is indeed OK to do it.
This question is about how to do it so to maximize the chance of success. I know about one postdoc that has several important papers published in the subject. I watched one talk he gave online and he seems very approachable. Moreover in his talk he showed very clearly that there are lots of things to be done in the field.
That said, I would like to send him a message and ask if I could collaborate with him remotely. What I want is to get involved in the research in this fied, help and gain experience.
A first point is that he doesn't know me, nor do I know anyone who knows him. To be honest I don't know anyone who knows people working in the field.
In that scenario, what is the best way to propose such collaboration and how much information should I send with my message?
Decide on what this field means to you and act accordingly.  From your post, you are a PhD student and therefore presumably already have a field of study.  If this is your first semester it will be a simpler conversation than if this is your sixth.  I'm going to assume that you don't want to change your current field of research, but would instead like to work in both fields to some capacity.
Whether your current field and this new one are related, and the extent to which they are related, is also important to consider.  If they are basically unrelated, or only tangentially so, then you'd be asking this post-doc to train you to be a collaborator.  If they are related well enough, then you'd be a useful collaborator to start with.
I personally would not begin a conversation around collaboration.  You appear to be too new to the field to do that.  Instead, I would begin by contacting the post-doc about his research, or potentially his talk, and discussing those open problems.  Are there any open problems that interest you and the post-doc, and in which you feel you could meaningfully make a contribution?  Start talking about your ideas, discuss it as though you are curious in a deeper way than someone who just watched a talk, but still not ready to jump ship and switch fields.
Through those conversations, you'll gain the insights you need to start proposing relevant questions for further research, and from there be able to ask this post-doc about collaboration opportunities.  Remember, this person also has their own projects.  They probably won't just bring you on to their own because you sent an email.  If you can both work on a truly new contribution though that would further your mutual careers, and if this doesn't negatively impact either of your abilities to get done what you need to do already, then it's a better conversation.
So, TL;DR: don't ask to collaborate, just start talking about a mutual interest, and see where it goes.