12399 ÷ 7 = 1771 R2
We're going to perform division on 12399 with 7 as the divisor.

Step 1:
7 goes into 1 0 times with a remainder of 1.
Write down 0 as next digit of of the result. 
Result so far: 0
Subtract 0 from 1 to get 1.
Bring next digit (2) of the dividend behind the 1 and repeat the process: 12 / 7

Step 2:
7 goes into 12 1 times with a remainder of 5.
Write down 1 as next digit of of the result. 
Result so far: 1
Subtract 7 from 12 to get 5.
Bring next digit (3) of the dividend behind the 5 and repeat the process: 53 / 7

Step 3:
7 goes into 53 7 times with a remainder of 4.
Write down 7 as next digit of of the result. 
Result so far: 17
Subtract 49 from 53 to get 4.
Bring next digit (9) of the dividend behind the 4 and repeat the process: 49 / 7

Step 4:
7 goes into 49 7 times with a remainder of 0.
Write down 7 as next digit of of the result. 
Result so far: 177
Subtract 49 from 49 to get 0.
Bring next digit (9) of the dividend behind the 0 and repeat the process: 9 / 7

Step 5:
7 goes into 9 1 times with a remainder of 2.
Write down 1 as next digit of of the result. 
Result so far: 1771
Subtract 7 from 9 to get 2.

The final result is 1771 with a remainder of 2.