General considerations
It is a very good idea to use a separate project in JIRA for QA tasks. They are somewhat related to dev department, but it is easier to manupulate security and workflow if you group dev issues in dev project, and qa tasks in qa project.
Test case as an issue
Saying that, you will just create an issue with type "Task" in QA project. You may also have already created a separate issue type "Test Case".
After that, fill Description with what you need to do in your test case. It is pretty much convenient - you can use tables, different fonts, pictures, links, etc.
When your test case is ready, assign it to a responsible person. Also put links to other issues, say, bugs (relates) or other test cases.
Also use as many custom fields as you wish to describe your test case.
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