This is the simplest technique and should be the first approach you try. Simply, if a given row, column or subgrid already has 8 digits filled then the last remaining digit will be the only digit not yet used. Try mentally counting from 1 to 9 to identify the missing number - with practice you'll be able to fill in the missing number without counting.
You can also use this technique without first filling in notes for each candidate so it's a great approach to use before adding any notes.
In the example, the number 2 is the only number not yet used in the middle column, and 5 is the only number not yet used in the top-left subgrid.
| 1 | 4 | 7 | 8 | |||||
| 2 | 8 | 6 | ||||||
| 6 | 9 | 3 | 1 | |||||
| 3 | ||||||||
| 5 | ||||||||
| 9 | ||||||||
| 7 | ||||||||
| 4 |
After filling in these numbers the middle column and top-left subgrid are now solved:
| 1 | 4 | 7 | 8 | |||||
| 2 | 5 | 8 | 6 | |||||
| 6 | 9 | 3 | 1 | |||||
| 2 | ||||||||
| 3 | ||||||||
| 5 | ||||||||
| 9 | ||||||||
| 7 | ||||||||
| 4 |