So, you see it really is quite simple! Instead of re-creating the wheel I’m posting a link to ready.gov to see their full list of home emergency preparedness. It tells you exactly what you need for the amount of people in your family. We keep ours in a large plastic storage tub that is readily-accessible in our entry closet.
If you want, you can purchase an already assembled kit, and there are many to choose from. Here are some of the top ones I’ve seen (affiliatelinks):