I ordered this same tape, and had the same questions... it's quite confusing! If you're looking at the same company I used, they have simplified the process quite a bit.
The first thing is you need to make sure you get a thermostat. This will allow you to set your desired temperature and will allow the heating tape to turn off once it reaches that temperature. This is extremely important because this tape can sometimes get up to 120 degrees!
The next thing is to figure out how much flex tape you need. Almost all companies sell it by the foot, which is what the price reflects. So if it says $3, it means 1 foot is $3, 2 feet is $6. One large bin, such as one used for dubias, will be heated fairly well with 1 foot of tape. You need to put this tape on the outside, either on the bottom or the side. I, along with many others, recommend the bottom. You don't want the tape under the entire bin though, just 2/3 of it. Place the food and water above the 1/3 part that doesn't have tape under it.
Buy 1 cord with the clips, per strip that you'd like to use. For clarification- you can use a continuous strip of flex tape for like 15 ft I believe it is. But most people will use 1 or 2 feet per plug, because we don't have an enclosure that is 15 ft long. So for example, if you have 3 large bins: you would get 3 feet of flex tape, 3 cords with connectors, I'd say 6 ft of foil tape, and rubber insulating tape. You can use electricians tape or buy their pre-made square rubber adhesive insulators. The former is a lot cheaper.
Other than this, it'll come with instructions. This is just an overview. Let me know if you have any questions