Take away big numbers column by column, and learn to borrow
Start small, then level up!
A written method. Write the smaller number under the larger one with the digits lined up, then subtract one column at a time, starting from the right.
When the top digit is smaller than the bottom one, you take 1 ten from the next column. The top digit becomes 10 more, and the digit you borrowed from goes down by 1.
Add your answer to the number you took away. You should get back the number you started with.
With Picture Counting, which takes pictures away from 10 or fewer. Then 1-digit subtraction, and after that 2-digit numbers with borrowing.