Don’t do it yourself
You need the right equipment, something could go wrong, but really you need someone else, preferably someone with experience, to check it is positioned correctly and straight.
Getting a piercing often takes but a few seconds. Measuring up is what takes the time. Professional piercers do it for a living. Let them do the work.
Decide what you want
Try to find a picture of it, or the name. If you can’t, draw a picture. If it’s a variation of something, get as close as you can.