How to Make +Email a Cover Letter for Volunteering ?

I'm thinking about applying as a volunteer at a cat cafe and their requirements are to email them a resume and (optional) cover letter, so I'm planning to do both! I have my resume finished, but I'm stuck on making a cover letter since in school we skimmed through that. I only know a few basic things such as including your volunteering experiences, addressing it to 'whomever it may concern', keeping it 3-4 paragraphs, and knowing about the company, otherwise I'm completely new to this.

Is there anything important that I am missing or that I should include? And also would it be better to type it all on a doc and attach it or should I at least write something in the actual email? Any help is appreciated! :)

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