The Catechism for the Catholic Church says that missing mass on Sunday is a grave sin, unless there is a serious reason (like needing to care for others), or a dispensation is given by their pastor.
2181 The Sunday Eucharist is the foundation and confirmation of all Christian practice. For this reason the faithful are obliged to participate in the Eucharist on days of obligation, unless excused for a serious reason (for example, illness, the care of infants) or dispensed by their own pastor. Those who deliberately fail in this obligation commit a grave sin.
Say that a person wanted to get a dispensation from Sunday mass (perhaps they got a job at a restaurant that required them to work on Sundays, which is noted as something that may be necessary in 2187). How are they expected to go about getting this dispensation? Is it as simple as sending an e-mail (or otherwise communicating with their pastor)? Or is there a more formal process?