First of all, you need to decide for what purpose you are preparing a survival kit. Do you intend to make it for your home, your car, or any other place? If it is for your car, you will want a special auto survival kit. If it is for your boat, you will want a bucket survival kit to keep everything dry.
Now you need to determine how many people you will need to provide emergency supplies for. If you live alone, it may just be one person. However, what if your neighbours need help? You might want extra supplies on hand. Make sure you include your animals into your calculations. If you are buying emergency survival kits for a church you may need supplies for 200, 1000, or even 10,000 people.
Determine what items you will need to have at hand for your emergency survival kit. You will want food and sterile water, for sure. Other things you might need include a flashlight, first aid kit, batteries, whistle, dust mask, tools, blankets, ponchos, portable toilets and liners, light sticks, radio, waterproof matches, water shut-off tool, utility knife, duct tape, pry bar, emergency shelter, fire ladder, and first response chair.
You might also need cat food, dog food, and small animal food (and water for them). Don't forget about diapers, formula, baby food, and water for the children. Make a complete list of everything you will need.
Decide on how much of each supply you would like. 72-hour emergency survival kits are a good starting point. However, if a hurricane comes and you need to survive for a month or more, you would want to be prepared. Some people store enough emergency supplies to last a year or two. This is not a bad idea, but you should do whatever you can manage. Any preparation you can afford is a good thing.
Figure out where you are going to purchase and store your emergency supplies. Some stores offer emergency survival kits that are already pre-assembled; and they are even willing to customize the kits to your needs by allowing you to purchase additional supplies and having everything shipped at once.
If you are short on time, this may be a good way to go. Expense is something to think about, but many places/people may work with you on financing the supplies. If you want to gather the supplies on your own, make a list of where you will buy each item. Once you have purchased your supplies, make sure they are in a safe and protected place, someplace where they are not likely to be ruined.