You take the high temperature (or average or low; whatever you're going for), for every day of one year for a specific area/city, and then each row/stripe is one day.

Before you start you assign each temperature range a color. The hotter days tend to be in the reds, oranges, and yellows, the colder days in the blues and purples.

There's a lot of variation. You can make the range 10 degrees for a lot of the same color, or change every 3 degrees for a lot of different color changes. You can do stripes or squares. You can do the average temp of the city you were born in during your birth year (with a unique color for your actual birthday). Or the same idea for a kid's or friend's birthday year. You can do the current year, in the city you're in, for a snapshot in time. (This also means it can't be finished until the year is over, lol.)