This is a strategy I use to teach my 2nd graders how to add two 2-digit numbers.
After learning about place value, students should understand the value of each digit. For example, they would know that 26 has 2 tens and 6 ones and can be represented as 20 + 6. Using this knowledge, students can decompose 2-digit numbers, add the tens together for each number, then add the ones together for each number, and then join those sums together. By being able to decompose numbers and then compose them as new numbers, they will eventually be able to use that strategy to mentally add two 2-digit numbers.
This product contains five sheets using five different math problems, and a 6th blank sheet so you can fill it in with your own math problems.
I used examples like this for modeling, guided practice, as well as exit slips to assess student learning. This strategy is just an introduction and a visual aide to help them see what they should eventually be able to do mentally.