Start with a warm-up of 400-600m low-frequency freestyle.
Next, you could move on to some technique drills.
Decide the strokes you want to practice with the drills below and mix and match however you want, so long as they add up to 400m in total.
For freestyle, try swimming with single-arm freestyle drills:
- Try swimming with either your left or right hand first while putting the other hand at your hip or at the front for 25m
- Change your hand and do the same for another 25m
For butterfly stroke:
- Start with 5 strokes of the left hand
- Repeat with 5 strokes of the right hand
For breastroke, try swimming with the 3-2-1 breaststroke legs drill:
- Hold your breath while kicking 3 breaststroke legs, followed by a full stroke
- Then, swim 2 breaststroke legs, followed by a full stroke
- Lastly, swim 1 breaststroke leg, followed by a full stroke
After you have finished, go for 10 - 12 laps of 25m sprints. With every 25m reached, swim slowly to cool down yourself down at the other side of the pool before starting again.
It is better to set a time for yourself to complete the entire programme.
For example, you could set a 1-minute time limit for each sprint (including the cooldown) to keep your heart rate or an overall 30-minute time limit for the entire programme.