For this recipe, I would suggest using avocados that are ripe but still slightly firm. This will help you dice the avocado into even cubes and it not to brown as quickly in the salad. To tell if an avocado is ripe, gently squeeze it. It should feel just slightly squishy. If your avocado isn't quite ripe yet, you can place it in a window seal or in a brown paper bag with a banana to help it ripen quicker. Or check out this great article for more ways to quickly ripen an avocado!