i applied to a school online and they said i could send in an additional essay if i wanted detailing my extracurricular and leadership opportunities. i'm not quite sure how to format this essay and how to brag adequately about my achievements. any suggestions?
I would list all of the activities you've done throughout high school, especially if they are activities that you have partaken in since an early age (music, sports, etc.) When talking about academic/athletic/extracurricular achievements, don't be afraid to tell them how much you succeed. Tell them what you've achieved, what positions you've held (varsity athletics, Section Leader, whatever) or what advanced classes (AP, IB, H) you've taken/are taking and GPA.
its okay to brag, they want that
Be careful about that advice; everyone brags. A top-4 University advised me they want to know what you can bring to that SPECIFIC university; they don't care that you think you are just as wonderful as the next applicant. Take the time to get to know them. Find out why THEY are special. Tailor your application/essays to their needs and how you can help them.
Focus on your achievements they speak for themselves.
Many students are worried about sounding conceited in essays. My advice to you is that you can't go wrong talking about your own accomplishments. As long as they're accurate accomplishments that you've earned over the years or truthful evaluations of yourself, it isn't REALLY considered bragging. Bragging would be making grandiose statements out of the blue (e.g. "I'm the BEST applicant you will see this year" or "Everyone is always telling me how great I am"). These sort of statements are subjective and don't hold much ground in the admissions department. Just be truthful and proud, without being conceited.