From combination to position
A combination tells you which digits a cage contains, but not where they go. Use rows, columns and boxes to determine the order. A digit already in a column cannot appear there again. Then fill the entire cage.
Worked example
A cage has two cells and total 3. Its combination is 1 + 2. If one cell’s column already contains 1, that cell must be 2 and the other cell must be 1.