You convince a Canadian Border Agent that they should let you in to Canada.
You meet the eligibility requirements and have the proper Border Crossing ID.
If you want to play games at the Border be warned Border Officials Do not play nice on both sides of the Border. http://www.cic.gc.ca/english/visit/apply-who.asp
If you are a US citizen, you can use a passport, a passport card, or an enhanced driver's licence. If you are a citizen of some other country, you may need something else, depending on what country you are a citizen of.
ID to show what country you are a citizen of. Passports are good, but, Americans can be let in with a driver's license, or similar (they need passports to return home though). No criminal record. And, that's about it! The process at the border usually takes about a minute. It is pretty easy.
As long as you have the required ID (passport card/enhanced drivers license/passport), don't have a criminal record, and don't tell the border guard something stupid that will red flag you, you'll be able to cross the border.