Add big numbers column by column, and learn to carry
Start small, then level up!
A written method. Write the numbers one under the other with the digits lined up by place value, then add one column at a time, starting from the right.
When a column adds up to 10 or more, you write the ones digit and carry the ten over to the next column on the left, where it is added in.
Line up the decimal points first, then add column by column as usual. The point in the answer goes straight under the other points.
With Picture Counting, which adds small groups of pictures up to 10. Then 1-digit sums, and after that 2-digit sums with carrying.