Here are the corrections for the sentences:

1. **Incorrect:** Today we play more badly than we did yesterday, so we need to practise more.
**Correct:** Today we are playing worse than we did yesterday, so we need to practise more.

2. **Incorrect:** I have never seen a most colourful kite than this one.
**Correct:** I have never seen a more colourful kite than this one.

3. **Incorrect:** Maria and her sister talked loud than the other guests.
**Correct:** Maria and her sister talked louder than the other guests.

4. **Incorrect:** John ran more fast than all the other competitors in the race.
**Correct:** John ran faster than all the other competitors in the race.

5. **Incorrect:** Please speak clearer because I hardly understand the instructions.
**Correct:** Please speak more clearly because I hardly understand the instructions.