Okay first of all to get started you should know how to add tens places right (10+10, 20+10, 30+20, etc.) Easy right? Now you need to know how to add ones (1+1, 2+2, 3+5, etc.) So now in a big question like                         

45                                                                                                                                                 24                                                                                                                                                 35                                                                                                                                                 96                                                                                                                                                 +21                                                                                                                                                ——     

You may think this question Will take quite some time but it won’t because I’m going to teach you an easy way. First add the tens places (the numbers on the left) the sum is “20″ right? Those numbers are easy to add, now add the ones (the numbers on the right) the sum is “21″ now since “20″ was the tens answer it will be just “2″ on the left and “21″ was the ones answer so it will go on the left making the answer “221″ did you get that? lets try another.    

26                                                                                                                                                  31                                                                                                                                                 13                                                                                                                                                 +25                                                                                                                                               —— 

So now the numbers on the left are 2, 3, 1, 2. Those added up are 2+3=5+1=6+2=8, the numbers on the left are 6, 1, 3, 2. Those added up are 6+1=7+3=10+5=15. So now we have 8 and 15. now since 15 is supposed to be on one digit it will give its tense place digit to the eight making it nine and 15 to 5, now put the numbers together and you have 95!!

Here’s how to works, since in the ones column it can only have one digit, so if it has two, then the tens digit will go over to the other number making it that much higher. Hope this helped, you will need to know a little about math to understand this though.