We're dividing 423339 by 21 
Our goal is to divide 423339 by 21.

Step 1:
21 goes into 4 0 times with a remainder of 4.
Record the quotient 0 as the next digit in the result. 
Result so far: 0.0
Subtracting 0 from 4 leaves us with 4.
Grab the next digit (2) from the dividend, add it to 4, then carry on: 42 / 21

On to step 2:
When dividing 42 by 21, we get 2 with a remainder of 0.
The next digit of our result is 2. 
Result so far: 2.0
If we subtract 42 from 42, we get 0.
Bring next digit (3) of the dividend behind the 0 and repeat the process: 3 / 21

On to step 3:
When dividing 3 by 21, we get 0 with a remainder of 3.
Put 0 as the next digit of the answer. 
Result so far: 20.0
If we take 0 away from 3, we end up with 3.
Include the next digit (3) from the dividend after 3, then repeat: 33 / 21

On to step 4:
33 divided by 21 is 1 with a remainder of 12.
Put 1 as the next digit of the answer. 
Result so far: 201.0
Subtract 21 from 33 to get 12.
Bring next digit (3) of the dividend behind the 12 and repeat the process: 123 / 21

Let's proceed to step 5:
21 goes into 123 5 times with a remainder of 18.
Write down 5 as next digit of the result. 
Result so far: 2015.0
Deduct 105 from 123 and we're left with 18.
Grab the next digit (9) from the dividend, add it to 18, then carry on: 189 / 21

Let's proceed to step 6:
When dividing 189 by 21, we get 9 with a remainder of 0.
Use 9 as the next digit of our solution. 
Result so far: 20159.0
Subtract 189 from 189 to get 0.
Since there are no more digits in the dividend, we add a zero to the remainder making it 0, and continue the process.

Advancing to step 7:
21 can be fit into 0 0 times, resulting in a remainder of 0.
Record the quotient 0 as the next digit in the result. 
Result so far: 20159.0
Subtract 0 from 0 to get 0.
Since there are no more digits in the dividend, we add a zero to the remainder making it 0, and continue the process.

Going ahead to step 8:
If we divide 0 by 21, we get 0 and a remainder of 0.
The number 0 becomes the next digit in our result. 
Result so far: 20159.0
If we subtract 0 from 0, we get 0.
Since there are no more digits in the dividend, we add a zero to the remainder making it 0, and continue the process.

The final result is 20159.0 with a remainder of 0.0.