What does write properly or don't write at all mean?

Answer 1

If you cannot write correctly, then you are not able to communicate by writing correctly. So it is better to not write all , in order not to confuse the reader.

Learn to write correctly, by forming the letters correctly. Don't allow 'a' or 'o' etc., to remain 'open' ; this makes the letter look like a 'u'. There are many other examples.

To aid correct writing, use a fountain pen, or a dip nib pen, or even a goose quill. Modern biros allow the hand to 'slip' and there by not correctly form the letters.

Answer 2

It means if you cannot use proper grammar and spelling, don't bother to write anything.