You can adopt the following two strategies:

1) Find the only possible position of a number in a group, a row or a column, for example:

In the rightmost group, consider the only possible position of the number 9:


Since the number 9 has already appeared in the first two rows, the only possible position of a 9 in this group is the remaining blank in the third row:



2) Find the only possible number that can appear in a blank, for example:

Consider the highlighted blank:


The number cannot be 2, 3, 4 or 5 as these numbers already exist in the same group, and it cannot be 6, 7, 8 or 9 either as these numbers are already in the same row. Therefore, the only possible number is 1: