What is a maxim?
Jens Timmermann argues in his not translated book "Sittengesetz und Freiheit" (DeGruyter, 2003), Chapter IV, that there are at least three different senses in which Kant uses the term "maxim".
The one important for the question is neither what could be called "basic principle", nor what could be called "higher order maxims" or "meta-maxims" (maxims that rule (the choice of) maxims). It is the simple sense of the particular subjective principle of a particular action (see Groundwork of the Metaphysics of Morals, Ak. 4:400, 420 fn).
Furthermore, according to Timmermann, every single maxim contains
- A particular situation
- A particular intent/end [Zweck] the action is aiming for
- The particular means/action for achieving the intended outcome (and thus what Kant calls "practical rules")
The mentioned example
In Kant's own words, he describes it as follows:
Another sees himself pressured by need to borrow money. He knows
full well that he will not be able to repay, but also sees that nothing will
be lent to him unless he solemnly promises to repay it at a determinate
time. (Groundwork, Ak. 4:422)
We can rather easily see how the maxim should look like: Being in financial distress without a perspective of being able to pay it back [Situation], I shall give a false promise [means] in order to nevertheless get money [intention/end]. Kant himself gives the following formulation:
When I believe myself to
be in need of money [Situation] I shall borrow money, and promise to repay it, [means] even
though I know that it will never happen [situation] (ibid)
As you can see, the actual intent/end of getting money is only implicit in this case, but should nevertheless always be taken into consideration when it comes to fully fleshed out maxims.
I would argue that your examples are not proper maxims at all, and that's why it is hard to universalise them per application of the Categorical Imperative. They lack situational and intentional dimensions.
Regarding universalisation in general
Henry Allison argues in his Kant's Groundwork for the Metaphysics of Morals: A Commentary, Chapter 9 (Oxford UP, 2011) that in order to correctly understand the Categorical Imperative, we have to apply a certain mode of universalisation. He further argues (and I strongly disagree with him here), that the original formulation would only include intra-subjective universalisation, while the Formula of Autonomy would necessarily include inter-subjective universalisation.
Intra-subjective here means that we explicitly keep our subjective standpoint and reason from our feelings and knowings, not abstracting from ourselves. Inter-subjective, on the other hand, means that we explicitly take into consideration the conceivable needs, thinkings, and positions of others, essentially not asking "can I", but "is it possible to think at all".
I think that the term inter-subjective universalisation is really good to get a first feeling of how it works. It is not about what the particular, empirical you thinks or feels things should be like (a major difference to the Golden Rule!).
On the other hand, I also think that by implying that there even is such a thing as "intra-subjective universalisation", Allison misinterprets the whole argumental arc of part two of the Groundwork at this point since the second section is analytical and therefore all that happens in later formulations, supposedly allowing for an "inter-subjective universalisation" not contained in earlier formulations, is an explication of what is contained in the original formulation. Well, at least Allison is not clear in distinguishing between what Kant wanted to say and what he thinks is actually written at this point.
Explaining Kantian universalisation
But how can we understand this form of universalisation? It means that we basically have to put ourselves into the shoes of every single rational being (in the Kingdom of Ends) and, using this perspective, decide whether our maxim is morally acceptable or not.
To make this a bit more explicit: At the same time we have to imagine
a) that every single person would in this situation necessarily act the same way our maxim proposes (as per the Formula of Law of Nature) and
b) that we have to respect the dignity of every single person in our decision, always treating them as autonomous agents, never as mere means (as per Formula of Humanity). This does explicitly not exclude using people as means, otherwise being an employer could end up being immoral.
In a second step, considering these two aspects, we will see whether there
a) already is an inner contradiction in this thought ("it cannot even be thought") - For example, the false promising would destroy the whole social instrument of promising as nobody would believe in promises anymore if we knew that everybody will necessarily lie the moment he thinks to be in trouble. But the maxim relies on that very presupposition (see comment below as well). So there is an inner tension (or contradiction) in trying to make false promising a (necessary!) general law for everybody. Because for making the promise (or communication in general) work it is essential that people generally believe in what you are saying. But a law like that would undermine the credibility of such utterances. That means a principle like this as a general law violates logic and is in this sense "unthinkable" - coherently.
b) looking at how society would end up like if everyone would necessarily do it the way you do cannot be wanted by you (read: as an empathetic, rational being! Sado-masochists could be fine with everyone slapping random persons for sexual stimulation, but this is not what we're talking about!). This account is insofar not trivially consequentialist as it is completely irrelevant what the actual consequences of your actions in any particular cultural or historical context would be. All that counts is what a society of sentient, rational, and potentially morally perfect beings would end up like (abstracting from specific cultural and situational circumstances) if your maxim would become one of its general laws. This has been one of the main sources of both criticism and praise of his deontic morals.
This is basically the argument he presents over the course of the second part of the Groundwork, summarised in 4:435-40.