How do you tell someone you have alterous feelings for them?
The same way you would tell someone you have any kind of feelings for them. Be honest. Do it for you, not for whatever hypothetical outcome you're hoping for. I would suggest that if they don't know what alterous feelings are, focus more on describing your specific experiences rather than using a label. Do you enjoy the time you spend together engaging in shared hobbies? Do you like certain aspects about their personality? Do you want to get to know them on an even deeper level? Let them know.
And, if you're looking for a formalized relationship of some type, be prepared to accept rejection. I'm not saying you will inherently be rejected or to expect rejection; I am saying that if someone says that what they want doesn't align with what you want, you need to be willing to accept it. To be vulnerable is to open yourself up to the possibility of getting hurt.
