I am going to give an example then explain how I added it. :) Ok, so say you have to
11
add 246+396 246
+396 so first add 6 +6....you get twelve. And because it is a double digit, you put the 642 under the 6 and then the one over 4. Next add 4+9+1...you get 14 so you would do the same as you did with 12 but instead with the new sum..14. Now you have to add 2+3+1 which is 6...and because it is a single digit, you place it under the 3 and get you answer of 642.
So I added this by starting on the last number and adding from right to left by one set of numbers at a time.... I hope this helps!