Filing Your Basis of Claim Form
Laying out your claim as well as you can within 15 days. If you filed at an airport, you were given a "Basis of Claim Form" from the Canada Border Services Agency once your claim was referred to the Immigration and Refugee Board. You have 15 days from the day your claim was referred to file with the IRB, regardless of whether or not it was filed on a weekend.
Filing Your Claim
Once your claim is as good as you can get it, you are required to file your claim within fifteen days of your claim being referred to the Immigration and Refugee Board.
File at the IRB office
Filing in person at an IRB office is the best option, if you can swing it. It gives you the advantage of being able to file the claim the same day and to have the stamp from the local office on your copy of the Basis of Claim form. Make sure that when you leave the IRB office, your copy of the basis of claim form has a stamp on it from the Immigration and Refugee Board with the date you submitted it. You'll also get your case number with the Immigration and Refugee Board at this time.
Mail it in
I do not recommend this approach, but if you must mail it in, do so with a courier service with a guaranteed delivery window. Do not rely on Canada Post to deliver it on time, unless you're getting Priority service. Make sure you keep note of the tracking number. If you're mailing it in, mail it to the IRB office for your region. The address should be on the Notice given to you at the airport by the CBSA.