Relics were, as you rightly identify, a major part of the cult of saints. They could be formed either of the bodily remains of a saint (often a whole body, but limbs, digits, heads, blood, etc would all serve) or by what are called ‘contact relics’ which are things the saint touched or used – clothes were the most common, but we also find bowls, cups, sticks, as well as manufactured contact relics, made by placing an object in or on the tomb of a saint to absorb their holiness, and then used as a relic. These last were often brought to the tomb by pilgrims and then taken away with them. Relics were held to retain a vestige both of the power of the saint, but also of their individual self, and thus were treated as essentially living people – they were petitioned, people gave them gifts, etc. The distinction between the bodily remains of the saint and the individual person were fuzzy, at best. Having relics of a saint within a community was a way to manifest the power of the saint, and ensure that the ‘virtus’ (here meaning both virtue and power) could be used for the benefit of those possessing the relics.

So, how did people go about getting the relics in the first place? Well…..pretty much the way you think. Andre Vauchez, still the pre-eminent historian on medieval sainthood, describes it thus:

‘Hardly had a servant of God breathed their last than a series of macabre operations began: the opening up of the corpse, removal of the viscera to be placed in an urn, deposition of the remains in a temporary reliquary…’

Sometimes this was undertaken by the community in which the saint had lived, especially if they were a monk or a nun – Clare of Montefalco (d. 1308) claimed to have the instruments of Christ’s Passion embedded in her heart (literally). When she died, the sisters of the nunnery opened up her body to check, and separated out her heart as a specific relic. Sometimes there was contention over who should be first in line for the relics, as with the death of Francis of Assisi (d.1226), where the township of Assisi recovered his body from Bagnara, where he died, and brought it back to Assisi under armed guard, for fear that the Franciscans should attempt to steal it for relics of their own.

Saints were aware of this practice, and sometimes commented on it while they were alive. Lutgard of Aywieres (d.1246) was a holy woman active in the Low Countries in the thirteenth century. Her biography was written by Thomas of Cantimpre, who was borderline obsessed with beguine holy woman such as Lutgard, and spent a great deal of time with her while she was alive. At some point, he asked the Abbess of her community for Lutgard’s hand as a relic after she died, which the Abbess granted, but subsequently mentioned to Lutgard, who promptly teased Thomas about why he might want such as thing:

“I have heard, dearest son [fili carissime], that you are already planning to cut off my hand after I die. I cannot imagine [multum miror] what you plan to do with my hand!”

Thomas, blushing [rubore perfusus], responds, “I believe your hand would be good for my soul and body, if I manage to get it, as I intend [si eam consequar, ut intendo].” Lutgard attempts to talk Thomas down to a finger, but he sticks to his original aim. After Lutgard dies, the Abbess rescinds the offer of a relic until Thomas agrees to write the Life of Lutgard, after which he obtains her hand. We see here that the saint has very little control over the disposition of their body after their death, and that relics are used as a sort of currency of the cult – the Abbess is prepared to part with the valuable relic only in exchange for a text which will ensure the spread and vitality of Lutgard’s reputation.

In terms of the ‘official line’, this is where we start to get questions of authority and power. One is, after all, only a saint for other people (in the sense that sainthood is an external, socially recognised quality), and exactly who those people were was very much the determining factor when comparing the process of relic creation. We have seen that for Francis, there was conflict and competition over who got his relics, and the means of exercising control over this was an armed guard. For Lutgard, the Abbess is able to assign relics as she finds useful, and clearly maintains strict control over the distribution of Lutgard’s body. If the saint is primarily considered saintly by a religious foundation or community, the relic creation will likely be careful and measured, with the saint’s body divided into appropriate parts, and displayed in a formal liturgical setting. If, however, one is considered a saint by a much wider section of the population (see Francis), the process of relic-making is going to be much more rough and ready, as well as subject to disruption and discontent, as speed and possession are the primary concerns.

We can find an example of this in the treatment of of Marcolinus of Forli in 1397, who was held to be a saint by the people of Forli, but not by the Dominicans of his order. This led to numerous confrontations, where the Dominicans tried to quietly bury Marcolinus’ body in order to prevent a cult (with its associated relics) developing. Eventually, the townspeople resorted to violence and forcibly opened the tomb, hoping to transfer the body to a more fitting location. At this point, it was found to be whole and uncorrupted, which in turn led to numerous miracles at the new tomb, and the acceptance of Marcolinus as a saint. The struggle over the control of Marcolinus’ body forms the central element of the question of authority – who has the power to determine if he is a saint, and on what basis? Ultimately, possession of the relics tips the scales, as it allows the people of Forli to gather the evidence of miracles associated with the relics, and thus prove Marcolinus’ sanctity. As it was the laity who considered Marcolinus a saint, the relics were created in a far less formal manner, and we do not get the measured and careful acquisition that we might expect from a religious foundation.

The point, really, is that there was no one way of making relics. We have instances where a saint’s body is practically ripped apart by a crowd all looking for their own piece of the saint. We have cases where there is a careful post-mortem deconstruction of the body in order to parcel the relics out fairly. In still other instances, the body of the saint serves as currency for the community, and is hoarded and doled out in exchange for other services. The papacy and the institutional Church mandated no formal means of creating, displaying, or using relics in the medieval period. The multiplicity of ways of making and using relics are largely the product of dialogue between the clergy and the laity, and a reflection of the complex ways in which everyday religion came into contact with theological understandings of sanctity and the power of the saints.

This is a bit of a brief overview, taking only a handful of examples. If there's something more specific you're interested in, please do let me know and I'll see what I can dig up.

Sources

André Vauchez, Sainthood in the Later Middle Ages (Cambridge: University Press, 1997).

Rachel Smith, (2016) 'Language, Literacy, and the Saintly Body: Cistercian Reading Practices and the Life of Lutgard of Aywières (1182–1246).' Harvard Theological Review, 109(4), 586-610.

Aviad M. Kleinberg, Prophets in Their Own Country: Living Saints and the Making of Sainthood in the Later Middle Ages (Chicago: University of Chicago Press, 1997).

Patrick J. Geary, Furta Sacra : Thefts of Relics in the Central Middle Ages. (Princeton: Princeton University Press, 1978)