For a small portion enough for 2 or 3 people, I use three large ripe avocados. Just use a fork to smash them in a bowl and add a tablespoon of lemon juice and a tablespoon of mayonnaise. Mix well.
Add a little salt to taste and refrigerate until chilled. I serve it with potato chips or use it instead of mayo on sandwiches. This also goes really good on tacos.