That is a very, very big question, and would require far more space than the 3500 characters available in this forum. But here are some ideas to get you started.
If you are basing this critique on Kant's work Foundations of the Metaphysics of Morals there are several points at which a criticism can begin. The most basic tenet, with which Kant begins the work, is that a system of morals must be based on ideas derived both from a priori reasoning -- that is, derived from ideas of principles or ideals that are simply assumed and accepted -- and ideas derived from experience. With this idea, to begin, many people could take issue that it would be this a priori/empirical synthesis at all -- you could effectively argue for an all-a priori or an all-empirically derived system. There are examples in history, and by various philosophers (contrast Plato and Nietzsche, for example,) of each kind of system. Before even delving into the particulars of what Kant believed, you could take issue with his method. This alone could provide many pages of debate. For example, one might say "All morality is already known; what is good and what is bad does not need to be tested or proved, merely applied from a priori knowledge -- be that a religious system, "common sense", or the received laws of a community, or some other source." The counterargument could be "All morality must be derived from empirical testing -- we find out what works and what doesn't work for our community (or class, or caste, or family, or nation, etc) and then we apply our findings as a system of morality." Depending on the context, and the cases applied to each, effective arguments have been presented for both sides (as well as fractional ingredients from either side, as well.) Kant presents the other half of this debate in Critique of Pure Reason (also called Critique of Practical Reason) in which the supreme principle of morality is explained.
Another ripe argument is Kant's tenet that the worth of an action performed from duty lies in worth of the law or commandment. This idea was made famous almost two centuries earlier in Shakespeare's Henry V,
The King is not bound to answer the particular endings of his soldiers, the father
of his son, nor the master of his servant; for they purpose not
their death, when they purpose their services. (IV.i.)
In other words: it is not enough to be ordered (or even compelled) to do something. If the law (or lack thereof) on which the action is based isn't right, then the person doing the action bears the moral consequences.
It's unclear from Kant's book if this applies in all cases, but knowing Kant's fondness for absolutes, it would appear that this would apply to those compelled to do so (children, prisoners, conscripted soldiers, slaves, etc.) This is where many people find a critique of Kant to be fruitful. It can be argued that Kant is unnecessarily inflexible on these points.
Kant wrote two significant books on morality, and the bases of his morality are contained in many of his other works. In order to critique his morality it is necessary to go back to they way he formulated his ideas. Kant's morality, and, indeed, any morality, is an enormous subject, and it is probably best to either choose one aspect of it to criticize and analyze, or go to the philosopher's first principles.