Hello and thank you for your question.
There are many variants to do this. I give you a couple of examples:
1. Start with 5 minutes of breast stroke followed by 10 minutes of free style, 10 minutes of back stroke. Finish with 2 minutes of butterfly followed by 3 minutes of breast stroke. Repeat!
2. Lap one (one pool length): Back stroke. Lap two: free style. Lap three: butterfly. Lap four: breast stroke.
You can also make it more fun and challenging by treading water (eggbeater kick) which gets you out of breath when performing it very fast. You may make this even more challenging by holding a light weight above you head (an advance exercise).
I personally like to start slow (breast stroke) and finish strong (butterfly)
Make sure you stretch before and after, and rest as needed during your swimming and make it fun yet challenging.
I hope this will help,