You basically just ask the priest if he will do so. Practically speaking, however, this typically takes place through the offering of a stipend. (usually like 5 or 10 bucks) If you want to offer it at a parish, you just go to the office and talk to the secretary or whoever is available. They’ll help you pick a date for the Mass and everything. If you’re sending your offering off to some missionary order or something, then it might be handled a little more generally by just including the donation and listing your intention in the mailing or whatever.