How to distribute values evenly when having several independent variables?

I am doing a research study with teens. Previously, I have already recruited and studied, let's say, 11 teens (with various ages, gender, and ethnicity). Additionally, I am planning to recruit 10 teens. However, I need to try to go for diversity which means I need to distribute those 10 participants I want to recruit more or less evenly considering their gender, age, and ethnicity and based on the teens I have already studied. Is there any statistical methods that I can apply in this case?