line up 136 and 212 with 136 on top and 212 on the bottom.
136
+212
____
now add up the end numbers, 6 and 2. That's 8. Since 8 is only a single-digit number, it fits in the space. Write that down under 6 and 2.
Next, add up the middle numbers, 3 and 1. That's 4, it fits in the space, so you don't need to regroup here either.
Now add up the first number in each, and that adds up to 3.
The sum is the number you wrote down! It should look something like this.
136
+212
____
348
You can say that the sum of these numbers is 348, and you did not have to regroup because the sums of each digit was under 10!