Been here before. Its all about shadow footwork drills as fast as you can  few times a week. Doing a certain pattern of footwork.

You could do 2 shuttle pickups. Put 2 shuttles at 1 corner of court. Pick up 1, back to middle, out to side, put it down, go back to middle, get the 2nd shuttle, same again, back to middle and then side again so the 2 shuttles are beside each other. back to middle, go to side, pick up a shuttle, back to middle, then rear corner etc etc until you are at the start. It should take about a min or slightly more to do at full speed. Think mines was always about 70 secs ish. Do this 3 times!! Take 1min & half break between each 1.

You could jus do general footwork but time it. Do it as fast as you can. Do it in 20 sec bursts, 10 sec rest, 20 sec work, 10 sec rest and do this for 2 mins. As weeks go on, build this up to 10 mins. If you can do this for 10 mins, ul be fit enough for singles!!!!!! Trust me. I think I ended my training ere at 6/7 mins and my fitness was good for singles.

The killer which I hated was to pick a long long space. Say 400m and run this as hard as you can from the start. The hard run should take about 1min30 sec roughly, ul be exhausted by end. Have 90secs rest and do this 3 times. Of course each week increase by 1 set lol.

Include in your week 1 long easy and I mean easy run!! about 1 hr long. It should be so easy, you think its waste of time.

But if you want to get fit, get richard gibneys the ultimate badminton athlete by badminton life. Its very good. He gives you a schedule for cardio and weights also to get you fit for badminton and it works, ive done it!!