We have seen how do multiply a multi-digit number by a one-digit number but we didn't see the case of the result of a multiplication has two digits.
When you multiply 3 by 3 the result has only one digit (9).
You write it under the line.
After that, you have to mutliply 3 by 4.
As the result (12) has 2 digits you don't write it under the line.
In case of the result of a multiplication has two digits you just write the Units of this number under the line.
At the same time you carry the tens.
That mean that you write it somewhere at the top-right of the drawing.
After that you have to add the carry number with the result of the next multiplication.
Finally you multiply by the hundreds, the thousands...Each time you add the number previously carry.
When there is no more digit to multiply by, you read the final result under the line.