All you do is move the decimal point two places to the right. If there aren't enough places, fill in from the right with zeroes. You will never need more than two zeroes. For example: 1 is 100%; 0.5 is 50%; 0.25 is 25%; 0.1 is 10%; 0.01 is 1%; 0.005 is 0.5%; 1.234 is 123.4%; 0.1234 is 12.34%. If the decimal point is missing as in 1, it's actually to the right but invisible. So 1 is 1. or 1.0 really and when we move the decimal point two places to the right we have to fill in with zeroes: 100%. The decimal point becomes invisible again, because we don't need it.
To convert from percent to decimal you do the opposite: move the decimal point two places to the left and fill in with zeroes if necessary, as in 5% which becomes 0.05.