With this solution approach, which is well suited for filling the first fields, you should look specifically for numbers in the section, where already many numbers are located. By step-by-step exclusion of the numbers from 1-9 you will find in the best case only one possible number for the considered box. In any case, the possible amount of usable numbers can be narrowly limited. With this principle you are now working on all blocks in Sudoku that meet these criteria. In this way, step-by-step you can always fill out more vacancies.