Use half as many tablespoons of basil as you have of butter. So if you're using a whole stick of butter (8 tablespoons), then you'll need about 4 tablespoons of basil. If you are using salted butter, don't add any additional salt. It's too much. But if you're using unsalted butter, sprinkle in kosher salt about ¼ teaspoon at a time until you feel like it's right.
You could use a food processor to pull all this together, but I just use my hands.
In a medium bowl smoosh the butter together with the basil until the basil is evenly distributed throughout. If you're adding salt, smoosh it into the butter in the same way. Shape it into a log, or a circle, or get craaaaayzay and press it into those little molds that are cool shapes. Suuuper idea for a fancy schmancy dinner party. If you're shaping the butter into a log (as seen pictured above) wrap it tightly in plastic wrap and refrigerate.
Recipe by Sugar Dish Me at https://www.sugardishme.com/basil-butter/