How can a particular trade of modern times get a patron saint?
There is no process for getting a patron saint either in the past or the present, unless it be the patron saint of a particular local church, in which case it is done by the local bishop, clergy and the community of faithful, which in turn is confirmed by Rome.
Popes or the Vatican generally do not name patron saints of guilds or other topics such as forests, as the faithful often are inspired to have their own local patron saint.
How Pope John Paul decided to have St. Isadore as the patron saint of the Internet is unknown. How he became (by what individuals) the patron saint of computer programmers is equally unknown.
The earliest records show that people and churches were named after apostles and martyrs as early as the fourth century. Recently, the popes have named patron saints but patrons can be chosen by other individuals or groups as well. Patron saints are often chosen today because an interest, talent, or event in their lives overlaps with the special area.
The Catholic Encyclopedia explains how patron saints are generally chosen:
Choice of patrons
Down to the seventeenth century popular devotion, under the guidance of ecclesiastical authority, chose as the titulars of churches those men or women renowned for their miracles, the saintliness of their lives, or their apostolic ministry in converting a nation to the Gospel. Urban VIII (23 March, 1638) laid down the rules that should guide the faithful in the future selection of patrons of churches, cities, and countries, without, however, interfering with the traditional patrons then venerated (Acta S. Sedis, XI, 292). As during the days of persecution the most illustrious among the Christians were those who had sacrificed their lives for the faith, it was to be expected that during the fourth century the selection of the names of martyrs as titulars would everywhere prevail. But with the progress of the Church in times of comparative peace, with the development of the religious life, and the preaching of the Gospel in the different countries of Europe and Asia, bishops, priests, hermits, and nuns displayed in their lives lofty examples of Christian holiness. Churches, therefore, began to be dedicated in their honour. The choice of a particular patron has depended upon many circumstances. These, as a rule, have been one or other of the following:
(1) The possession of the body or some important relic of the saint;
(2) his announcement of the Gospel to the nation;
(3) his labours or death in the locality;
(4) his adoption as the national patron;
(5) the special devotion of the founder of the church;
(6) the spirit of ecclesiastical devotion at a given time.
Patron saints of local churches are chosen by the local bishop, clergy and faithful and not Rome!
The naming of a patron saint of a parish church must be chosen by the clergy and the faithful of the area in union with their local ordinary (bishop) and then approved by the Congregation for Divine Worship and the Discipline of the Sacraments.
Often the faithful of a specific region will hold a particular devotion to a particular saint, thus making the choice more easy. The bishop can not name the patron saint of a church without the aid and inspiration of local clergy and the faithful.
As an intercessor or advocate before God, the patron is a created person, such as the Blessed Virgin Mary, the Holy Angels, a Saint or Blessed. For the same reason, the Most Holy Trinity and the divine Persons are always excluded as patrons.
A patron must be chosen by the clergy and the faithful, whose choice must be approved by the competent ecclesiastical authority.* In order that they may carry liturgical effect, the choice and approbation require the confirmation of the Congregation for Divine Worship and the Discipline of the Sacraments, which is granted by decree of this same Dicastery.
The patron of a place is distinguished from the title of a given church; they may be the same but are not necessarily so.
When a new parish has been erected in place of several suppressed parishes, the new parish may have its own church, which, unless it is a new building, retains its existing title. Further, churches of suppressed parishes, whenever such parishes are considered as ‘co-parishes,’ retain their own proper titles. - Patron Saints of Churches.
Ultimately it is up to the local clergy and the faithful to propose the patron saint of a parish church to the bishop, who will give his approval to the motion
Nota Bene: Number 8 simply means that the title of a church may be of the Most Holy Trinity or one of the divine Persons; but since they are not Saints, Blesseds or Angels they are always excluded from being the patron saint of a church. In this case we refer to the parish as being associated with a particular divine mystery and the parish’s feast day will occur on the feast when the Church celebrates that particular divine mystery.
Thus in the end, patron saints can be chosen by any group of the faithful (Catholic guilds, etc.) at anytime and Rome does not have to be notified. However, for the patron saints of parish churches, the above rules must be followed.