What are the steps and permissions required to add more "patronages" to saints? I understand well enough the steps to go through to become a Catholic saint. And my understanding is that saints are "patron saints" of certain aspects of life and those patronages get assigned after the canonization. For example, Marguerite Bays was canonized last year and is listed as the patron saint of the Franciscan tertiaries.

What I'm less clear on is how older saints get assigned "newer" patronages. For example, Saint Isidore of Seville was canonized in 1598 but is the patron saint of the internet, which obviously didn't exist back then. So who gets to start and authorize the process of adding new patronages to saints that already completed canonization?
What are the steps and permissions required to add more “patronages” to saints?

Apart from having a group of faithful that invoke a particular saint for for a particular cause, there are no official rules or norms in order to establish a certain Catholic saint the patron saint of something in particular. 

In other words, the faithful remain free to add more “patronages” to particular saints as their own particular devotion, needs or circumstances deem it necessary.

On very, very rare occasions, the Holy See will name someone a patron saint for of certain cause or people. This actually happened to St. Thomas More:


  On 31 October 2000 Pope John Paul II declared More "the heavenly Patron of Statesmen and Politicians". More is the patron of the German Catholic youth organisation Katholische Junge Gemeinde. - Thomas More


As you noted in your question, St. Isidore of Seville, is the patron saint of the internet. This also is one of those very rare circumstances when Rome intervenes in the naming of a patron saint:


  In 1997 Pope John Paul II declared Isidore of Seville the patron saint of the internet. Saint Isidore died in the year 636, long before the first host-to-host ARPANET connection in 1969. But Isidore did try to record everything ever known in an encyclopedia that was ultimately published after his death.
  
  From The Telegraph:
  
  Saint Isidore wrote a 20 book opus Etymologies, also known as the Origins, in which he tried to record everything that was known. Published after his death in 636, it was for a thousand years considered the encyclopedia of all human knowledge.
  
  Written in simple Latin, it was all a man needed in order to have access to everything he wanted to know about the world but never dared to ask, from the 28 types of common noun to the names of women’s outer garments.
  
  It was a tool by those seeking wisdom much like the internet is used now. - The patron saint of the internet is Isidore of Seville, who tried to record everything ever known


With all this said, there still is the glooming question as to why a particular Catholic saint could become the patron saint of something that did not exist during the saint’s day. Generally speaking, there should be some sort of connection between the two, either on a personal level or on some sort of physical level.

This is what happened in the cases of both St. Thomas More and St. Isidore of Seville. The case of St. Thomas More is obvious! But in the case of St. Isidore of Seville the connection is a little harder to understand.

The Franciscan Media has this to say about this subject:


  An amazingly learned man, he was sometimes called “The Schoolmaster of the Middle Ages” because the encyclopedia he wrote was used as a textbook for nine centuries. He required seminaries to be built in every diocese, wrote a Rule for religious orders, and founded schools that taught every branch of learning. Isidore wrote numerous books, including a dictionary, an encyclopedia, a history of Goths, and a history of the world—beginning with creation! He completed the Mozarabic liturgy, which is still in use in Toledo, Spain. For all these reasons, Isidore has been suggested as patron of the Internet. Several others—including Anthony of Padua—also have been suggested. - Saint Isidore of Seville


Popes generally do not name patron saints as the faithful often are inspired to have their own local patron saints. 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. But it is a logical step after the internet patronage question was made.


  In 1997, Pope John Paul II decided that the internet could use a patron saint to guide Catholics in its proper use. He chose Saint Isidore of Seville (560-636), Doctor of the Church, and last of the Latin Fathers. His twenty-book opus (called Etymologia, after the subject title of one of the books), made him an easy choice. The word “etymology” was Isidore’s own coinage. It means “the study of origins.” Today, the term is limited to the history, or origin, of words. Interestingly enough, three other words that I know of owe their invention to saints: “utopia” and “integrity” to Saint Thomas More and “soliloquy” to Saint Augustine.
  
  Here is a prayer to Saint Isidore that should be said before logging in on the internet:
  
  Almighty and eternal God, who created us in Thy image and bade us to seek after all that is good, true and beautiful, especially in the divine person of Thy only-begotten Son, our Lord Jesus Christ, grant we beseech Thee, that, through the intercession of Saint Isidore, bishop and doctor, during our journeys through the internet we will direct our hands and eyes only to that which is pleasing to Thee and treat with charity and patience all those souls whom we encounter. Through Christ our Lord. Amen.
  
  Source: Patron Saint for the Internet, Isidore of Seville


It seems that St. Isidore of Seville became the patron saint of the internet because he try to catalog all known existing information into book form. If computers were available in his day, he would have used the internet!

On a personal note: There is no patron saint for recycling or those involved in recycling. I do a lot of recycling, but for years I did not have a patron saint for it. Then one day, I came across the following article: Relic of St Clement found in trash settles into Westminster Cathedral. Ever since that day, Pope St. Clement became my patron saint when I am recycling. There are no rules or norms forbidding my choice in patron saint! For myself.

The following may be of interest to some:


Carlo Acutis is on path to becoming patron saint of the internet