With lining them up
Explanation:
Say you have 357 + 872
You would position your numbers so that they line up then you add the numbers that line up with each other, if your number is 10 or more you need to carry the first didgit over above the next row if your number was 14 you would carry the 1 over above the next row then you would add that number to the multiplicated 2 other numbers.
357
+ 872
_________