We have numbers 990 and 323. Let's start adding them together.

Step 1: We'll start by adding the digits 0 & 3 in column 1 and get 3.

Step 2: We'll start by adding the digits 9 & 2 in column 2 and get 11.
We'll write down the last digit 1 and carry the 1 to the next column.

Step 3: We'll start by adding the digits 9 & 3 in column 3 and get 13.
We'll write down the last digit 3 and carry the 1 to the next column.

So, 990 + 323 = 1313.

