Here's a detailed explanation posted by MaG on an older thread:
There is an outside number in the square that is the same as the one on top.
That is the sum you are hitting for.
You have to enclose a set of numbers that will add to that.
Remember that you can also use the right number of a double digit as the end sum -
For example: you need to look for a set of numbers that will add up to 8.
You can look for a 4 and 4, or 3 + 3 + 2, or it can be 15 + 13.
After you have those set of numbers are enclosed in the square click the button at bottom right to enter that.
If correct, the enclosed numbers will disappear.
An alternative solution is to just click randomly around the box edges. It may take a little longer, but the end result will be the same.