Theory:

When a decimal part is multiplied by \(10, 100, 1000\), etc.,
it is enough to move the point by \(1, 2, 3\), etc., digits to the right  respectively.
  
When a decimal part is divided by \(10, 100, 1000,\) etc.,
it is enough to move the point by \(1, 2, 3\), etc. digits to the left  respectively.
Multiplication
  
\(19.23\) × \(10 = 192.3\)
The point moves \(1\) digit to the right
\(19.234\) × \(100 = 1923.4\)
The point moves \(2\) digit to the right
\(19.345\) × \(1000 = 19234.5\)
The point moves \(3\) digit to the right
  
Dividing
 
\(99.2345 : 10=9.92345\)The point moves \(1\) digit to the left.
\(99.2345 : 100=0.992345\)  The point moves \(2\) digits to the left.
\(99.2345 : 1000=0.0992345\)The point moves \(3\) digits to the left.
\(99.2345 : 10000=0.00992345\)
The point moves \(4\) digits to the left.
 
"If necessary, many zeros may be added or dropped before or after the decimal as they do not change the value."
Example:
\(0.2 : 100 = 000.2 : 100 = 0.002\)
 
\(0.2\)×\(1000 = 0.200\)×\(1000 = 200\)