Use a full stop to show a decimal point. For a decimal number less than 1, place a 0 before it.
Unemployment has fallen to 5.28%.
The Reserve Bank has cut interest rates by another 0.25%.
In general, show decimals to no more than 2 decimal places. Round down when the last digit is 4 or below. Otherwise, round up.
6.34 round down to 6.3
6.869 round up to 6.87
Of course, in specialist, technical and scientific documents, format the numbers to the level of detail needed. Consult a specialist scientific style guide for further detail if you need to.