An X-Wing occurs when a candidate appears in exactly two cells in each of two different rows, and those cells share the same two columns. The candidate must be in one of two diagonal arrangements, so it can be eliminated from all other cells in those two columns.
For each candidate value, look at rows where that value appears in exactly two cells. If two such rows share the same two columns, you have an X-Wing.
The candidate must appear once in each row. With only two positions per row, and those positions aligned in columns, the value must fill one diagonal of the rectangle. Either way, the two columns are accounted for, so the candidate can be removed from other cells in those columns.
Let's find an X-Wing. We will focus on digit 3. First, we identify rows where 3 can go in exactly two cells.