Fill a wine glass with ice cubes (I used about 6 ice cubes)
Pour in the aperol.
Top the aperol with the prosecco. Add a splash of club soda. Stir gently to combine.
Add a slice of orange to the glass. Enjoy!
Notes
Make sure your prosecco is chilled - the spritz will taste better and stay colder longer this way
Use a high quality prosecco - since prosecco plays a large part in this cocktail, using a high quality one is crucial. And most importantly, don't use white wine in place of prosecco! It just won't be the same.
Just add a splash of club soda - not too much or it will dilute the flavors.
Feel free to play with the aperol to prosecco ratio, but I find the 2:3 ratio is optimal. I also measure with a shot glass (with oz markings on it), to get the ratios just right.