Scanning means choosing a number and a box, and looking for the cell in the box where that number goes. For each empty cell in the box, look at its row and its column. If the number is already in one of them, the cell is ruled out. When only one is left, the number belongs there.
You don't need to write anything down. This is how almost everyone starts, even without knowing it has a name.
An example
We are looking for the 8 in box 5, the middle one. The box has six empty cells and no 8 yet.
There is an 8 in row 4, on the right. There is another in row 5, on the left. No cell of the box in those two rows can hold the 8.
Column 6 also already has an 8, higher up, in row 3. That rules out the last empty cell of the box in that column.
One cell is left: the one in row 6, column 5. That is where the 8 of box 5 goes.
Cross-hatching
What we did in the example has its own name: cross-hatching. Every number already in the grid crosses out its row and its column. Where the lines pass over a box, few free cells are left, sometimes only one.
Scanning with a method
One number at a time. Choosing a number and going through the nine boxes looking for it is faster than looking at everything at once.
Start with the numbers that appear most. A number that is already in the grid seven times only has two places left to fill. The lines rule out almost everything.
Scan rows and columns too. The search is the same (the cell where a number goes); only the unit changes.
After placing a number, go back. Every new number is one more line. It may solve a box that a moment ago still had two options.
When scanning finds the only place for a number in a unit, it has found a hidden single, the first real technique. When it stops finding numbers, it is time to write pencil marks.