How do you make a whole number a decimal?

1.0, 2.0, 3.0, 4.0, 5.0, etc. etc.
Assuming you mean a whole number rather than a hole number, you do not need to do anything.

A decimal version of a number is simply a way of expressing it in a form in which the place value of each digit is ten times that of the digit to its right. That is all a decimal is: no need for a decimal point or any 0s or anything else.

So, for example, 67 is the decimal equivalent for sixty seven.