The pointed nib has flexible tines, so whenever you apply pressure, the tines will spread apart and more ink will flow from the nib. This is done on downstrokes, these are strokes that go down, towards you. The weight of these downstrokes is controlled by how much pressure you apply.
Upstrokes are thin, no pressure is applied. You don’t do this because your nib would catch the paper fibre and snag when you push then into the paper. This happens very quickly with very pointy and sharp nibs.
Both down-and upstrokes should be consistent, so be sure to practice a lot. Practice of basic strokes is great for warming up, I have a practice sheet for basic strokes here.
You can start practicing downstrokes for half a page, and then upstrokes, then combine the two. Try to keep the strokes parallel to each other and keep the height and baseline consistent.