I first learned about grapefruit and tequila the summer I turned 21, when I worked at a resort on Lake Powell. They gave me a job in the restaurant at Bullfrog Marina, on the Utah side, about 70 miles from the closest town. In addition to college students like me, the people I worked with were quite a mix: local people who grew up working there, professional seasonal workers who traveled from summer places to winter ski resorts, and random people who needed something to do and somewhere to be.
Paloma, too, if you felt like using more ingredients.
For each 750 ml bottle, you need the peel of two grapefruits and a 1 liter bottle.
If the white on the back of the peels is transparent and porous, you should be OK. If it's thicker than that, pare some off with a small, sharp knife.
Put the peels in the jar and pour the tequila over it. Shake the jar gently and put in a dark, cool place in the kitchen.
Let steep for a week, then strain into another jar and enjoy! You know, if you're of legal age. If not, don't even think about it!
I'm currently enjoying a little with sparkling water and a wedge of grapefruit (see top photo). It's delightfully fruity without being too sweet.