Review your paper for spelling and grammar mistakes. Format it according to the journal's requirements. These may vary from one journal to another; review the journal's submission guidelines for specifics.
Submit the paper to the mathematics journal. While some magazines accept electronic submissions (via email), you may have to submit a hard copy through snail mail.
Revive the paper if requested. Before a paper is published, it is reviewed by someone with knowledge of the paper's subject for originality and depth. Suggestions for revision may be made during this process. Make the corrections and resend the paper. There may be several rounds of corrections.