When you ask how Karma is made you have to pick your reference point in Buddhism. I will pick the Awakening of Faith which tries to show how Karma can be possible by positing Storehouse Consciousness (Alayavijanya) as the place where the seeds of Karma are laid down and then persist to fructify later to produce the Karmic quasi-causality across time. Their answer is that Karma is made by laying down seeds (bija) in the Storehouse consciousness through ones actions and thoughts.
What is interesting is that we can model Karma with the Emergent Meta-system which is a complex dynamic made up of the normal system and the Special Systems working together. Fa Tsang’s commentary on the Awakening of Faith is the only book I have found so far which is aware that he is using the Emergent Meta-system as a model. Once we realize that Karma is a traditional concept of the Emergent meta-system dynamic then we can start to be very precise about how Karma must work. The Emergent Meta-system is a cycle of aggregates, or swarms of instances which move from seeds to monads to views to candidatesusing a series of operators that are creation, mutual action, schematization, andannihilation. This cycle produces something from nothing, which is what Stuart Kauffman calls order from nowhere in At Home in the Universe. It is a natural cycle we see in plants that produce seeds, that grow into trees (whose leaves are the monads) that then interact mutually through putting forth flowers to bloom that attract insects that can pollenate them, and that gives rise to fruits that then have seeds in them. A great analogy for this process also from China is a series in the game of Wu Chi (Go in Japanese).
So the ultimate answer for how Karma is made is that it is a natural cycle which is powered by a relaxation that moves from high energy to lower energy. In that cycle that acts like a genetic algorithm, various elements are made based on trace templates which have variation. These elements interact with each other and develop views in relation to each other that is the basis of mutual schematization, and this produces candidate variations of each other, that then annihilate to create seeds for the next round of the cycle. Karma can be equivalent to emptiness because it is in fact arising from nothing and returning to nothing. At each stage it only produces aggregates in swarms. Each of these sets of aggregates has its own emergent properties that come from the fact that hyper complex algebras lose properties as the cycle goes through its relaxation process. But interestingly the relaxation process produces anomalous high probabilities that the cycle will start up again, because it produces energy peaks locally even though there is local relaxation. More on the Emergent Meta-system can be found athttp://works.bepress.com/kent_pa… in Reflexive Autopoietic Dissipative Special Systems Theory and in Autopoietic Reflexive Systems Theory as well ashttp://nondual.net
I recommend An English Translation Of Fa-Tsang’s Commentary On The Awakening Of Faith translated by Dirck Vorenkamp