Act only according to that maxim whereby you can, at the same time, will that it should become a universal law.
Kant, of course, spends considerable time explaining how this should be understood. But let's suppose a scenario: you are a natural law theorist and don't think Kantianism is right, but you think there's a lot that is good in it. How do you 'translate' Kant into natural law with minimal distortion so as to establish that more rigorously in natural law terms?
Well, everything in natural law theory depends on two things: the theory of practical reason and the theory of law (or obligation). Because Kantianism also has the structure of a certain conception of practical and a theory of obligation, it will actually translate fairly easily.
So we start with law. Taking Aquinas's famous definition, law is an ordering of reason to common good as promulgated by one who is caretaker of the community. So suppose we understood "universal law" in a Thomistic rather than Kantian way? What's universal about it in Kant is that it applies to all rational beings. And a maxim is just the rule you make for yourself in a decision, so:
[1b] In your decisions, act only according to that rule that could be a rational ordering to good common to all who have reason.
But a more common way of saying this, rather than talking about maxims or rules in decision, would be simply:
[1c] Seek and do what is appropriate to what is common good for all rational beings.
But let's stop a moment and think about how we get precepts of natural law in the first place. According to Aquinas, the most general principle for reasoning about practical decisions of any kind is:
Good is to be sought and done, bad is to be avoided.
There's nothing inherently moral about this; it's the general principle which makes it possible to say that tying your shoes before running up and down stairs is rational. Whether the law of natural law comes enters into the question depends on the kind of good involved. Aquinas's definition of law requires common good -- note that this is literally what it says, good shared in common, and that it is any kind of good shared in common, not (as in utilitarianism) only good consequences. So the first principle of practical reason becomes the first precept of natural law when the good in question is the common good of human beings.
So if Kant's categorical imperative is understood according to the Thomistic account of what a law is, it is very like a close specification of the positive clause of the first precept of natural law. This actually makes sense. The first principle of practical reason is to practical matters what the principle of noncontradiction it is in theoretical matters: it distinguishes the rationally incoherent from the rationally coherent, and Kant's categorical imperative is entirely concerned with rationally coherent action. And the first principle of practical reason becomes a precept of natural law when it concerns the common good of rational animals, while the categorical imperative specifically concerns law appropriate for all rational beings, so if you add to the categorical imperative the Thomistic notion that law is an ordering to common good promulgated by its caretaker(s), you get something very similar to the first precept of natural law. They are functionally and structurally analogous to begin with.
You can do this, incidentally, with all the formulations of the categorical imperative. The Law of Nature formulation works the same way as the above. The End in Itself formulation converts when you add to the above the idea of human nature itself (our nature as rational animals) as being a necessary part of the common good of all human beings, and then you get a precept to seek what is appropriate to human nature. The Kingdom of Ends formulation can be converted by adding the idea that a kingdom or realm is constituted by friendship, and that human beings are caretakers for the common good of the human race, and then you more-or-less get a precept to act in a way consistent with friendship with all human beings.
Of course, Kant doesn't have a natural law theory. But by converting Kant in this way, it becomes more straightforward to sort out what a natural law theorist could regard as right and wrong, good or bad about Kantianism. And none of this is to claim that the converted precepts, as merely converted, are flawless or have the same role in natural law theory as the categorical imperative in Kant's account. But it is, in fact, of the very nature of natural law to seek what is appropriate to the common good of all human beings, and it is, in fact, a matter of natural law that we should seek the good appropriate to human nature, and it is, in fact, a matter of natural law that we should seek what is required in order to make possible friendship among all human beings. (The most interesting difference is that these are not all equivalent precepts in natural law theory, despite being all very general and exceptionless precepts. This is because Kant's formulations just use three vocabularies human beings use in moral matters to restate exactly the same categorical imperative, but the analogous vocabulary in natural law theory cannot cover exactly the same ground.)
You could do the same thing in the reverse, but this is less interesting, because from a natural law perspective, this is in a way what Kant did in the first place, and from Kant's perspective, all moral vocabulary can be conformed to the categorical imperative to show either its incoherence or its true form. Natural law theorists have tended not to be so bold.
Which is unfortunate, as it contributes to a common error in understanding natural law theory, that there are distinctive classes of 'natural law arguments'. If natural law theory is true, all good moral arguments are natural law arguments. You can no more have a distinctive category of 'natural law arguments' than you can have a distinctive category of 'arguments using the principle of noncontradiction', and for exactly the same reason. To be sure, you can have arguments that are more explicit about using it than others, but all good arguments are principle-of-noncontradiction arguments, and (if natural law theory is true) all good moral arguments are first-precept-of-natural-law arguments. Thus the natural law theorist should be able to say, of any moral argument, what in it is good in terms of natural law.