You don't. You DECLARE it, then use it, exactly the same as you might in an SSMS window.
Can you give us a few more details of what it is you are trying to do? Cursors are rarely appropriate.
For fast, accurate and documented assistance in answering your questions, please read this article.
Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden