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.
After filling in these numbers the middle column and top-left subgrid are now solved: