Kant himself offers ideas of how to apply the formula in Ak. 429-30. I will quote and parse the text in order to highlight the guidance he himself has given for this particular formula (translations from Kant, I. (1785/2011), Groundwork of the metaphysics of morals: A German–English edition (M. Gregor & J. Timmermann, Trans.), Cambridge, MA: Cambridge University Press):
1. The human being is not a thing at anyone's disposal
First, according to the concept of necessary duty to oneself, someone
who is contemplating self-murder Will ask himself Whether his action can
be consistent With the idea of humanity, as an end in itself. If to escape from a troublesome condition he destroys himself, he makes use of a person,
merely as a means, to preserving a bearable condition up to the end of
life. But a human being is not a thing, hence not something that can be
used merely as a means, but must in all his action always be considered
as an end in itself. Thus the human being in my own person is not at my
disposal, so as to maim, to corrupt, or to kill him. (Ak. 429, bolded mine)
Here, the punchline is "a human being is not a thing" as well as not being "at my disposal", but further specifics are not given beyond the "end in itself" idea. It is just clear that one should not see the human body itself and its abilities (e.g. working power etc.) as a mere mechanical factor just like any other physical object (similar to Heidegger's Dasein vs. Sein). So there are only hints so far.
2. One does not have the rights (or "principles") of others at one's disposal
Secondly, as far as necessary or owed duty to others is concerned,
someone Who has it in mind to make a lying promise to others will
see at once that he wants to make use of another human being merely
as a means, who does not at the same time contain in himself the end.
For the one l want to use for my purposes by such a promise cannot
possibly agree to my way of proceeding with him and thus himself contain the end of this action. This conflict with the principle of other
human beings can be seen more distinctly if one introduces examples of
attacks on the freedom and property of others. For then it is clear that
the transgressor of the rights of human beings is disposed to make use of
the person of others merely as a means, without taking into consideration that, as rational beings, they are always to be esteemed at the same
time as ends, i.e. only as beings who must, of just the same action, also
be able to contain in themselves the end* (Ak. 429-30)
Here, Kant gets more specific in two senses: First, he makes clear that it is about human beings being the (autonomous, but this is at a later stage of the argument) agents of their own principles and rights. And secondly, they must, as rational beings, always be able to see themselves as an (autonomous) agent of these principles and rights, given they knew all relevant circumstances of the situation. In other words: It should be reasonable that if they had the choice and knew the facts, they would be ok with you acting the way you do.
The latter formulation may lead to some misconceptions. The asterisk at the end of the paragraph leads to a very important footnote where Kant explicitly distances himself from the well-known Golden Rule ("Do unto others as you would have them do unto you.", see e.g. the Bible Tobit 4:15, Matthew 7:12 and Luke 6:31). The point here is that it is not about what you would like to be treated, but the other one, and only insofar they are a rational agent (since you cannot and should not guess what a particular person actually likes or needs to be happy, as most people do not even know it themselves, cf. Ak. 417-8). This solves alleged problems of the principle regarding S/M-people etc. which are prominent in literature as well as certain forms of enjoying prostitution and other problematic cases the Golden Rule is open for.
3. It involves acting for the betterment of all humanity
Thirdly, with regard to contingent (meritorious) duty to oneself it is
not enough that the action not conﬂict with humanity in our person,
as an end in itself, it must also harmonize with it. Now there are in humanity predispositions to greater perfection, which belong to the end of
nature with regard to humanity in our subject; to neglect these would
perhaps be consistent with the preservation of humanity, as an end in
itself, but not with the advancement of this end.
Here, the idea is that you have a moral responsibility for all humanity, i.e. you should work to advance all concerns of human beings to the best of your ability in all your actions, including widening your knowledge and abilities to the best of your natural features in order to be able to do so in the first place.
4. The same is true for the happiness and concerns of all others
Fourthly, as concerns meritorious duty to others, the natural end that
all human beings have is their own happiness. Now, humanity could
indeed subsist if no one contributed anything to the happiness of others while not intentionally detracting anything from it; but this is still
only a negative and not positive agreement with humanity, as an end in
, itself, if everyone does not also try, as far as he can, to advance the ends
of others. For if that representation is to have its full effect in me, the
ends of a subject that is an end in itself must, as much as possible, also
be my ends.
This is interwoven with the third point, as you will probably already have understood from my wording of the interpretation there read together with the quote. The idea here is the same: We have a duty to work for the better of all humanity since humanity is an end in itself (we are insofar we take part in being human/rational agents). The difference is that here, it is not just about doing everything to be able to do so by working hard to better yourself, but now also actively engaging in the betterment for and of others.
Kant himself highlights four aspects of the formula:
1) A human being (i.e. his body, life, working power, etc.) is not a mere thing that is at disposal.
2a) This is because every human is the agent of his own principles and rights.
2b) Hence, we must (negatively) respect the other human's ability to decide what he wants and treat him in a way we can reasonably expect a rational agent (not the particular person or yourself) to be able to agree with, given he knows all relevant facts.
3) Furthermore, we are (positively) responsible to work on ourselves to be able to do the best we can for the betterment of all humanity since this is what we rationally, as part of humanity, would and can expect from any rational agent.
4) In an extension of working on ourselves, our responsibility to do our best for all humanity applies to actively engaging with all humans (i.e. rational beings) and their concerns.
While the first two/three aspects are about negative duties (you should not) and more or less understandable and applicable insofar as they just exclude certain determinate ways of action, the latter two are a bit woolly and admittedly do not help much in determining a particular action.