What should we do to increase our stamina?

I'm not sure about badminton specifically, but for many other sports, people will run at a slower pace, for a longer distance, which makes the period that you're running longer. For example, instead of trying to get the fastest mile time, set a number of miles (such as 3) and keep a steady pace for the whole length. Play well for the entire game, rather than being fantastic at the start, and wearing yourself out for the end of it.