How to make some parts of a line smaller or bigger?
For example the middle of this line.


If I try with a the decreasing traingle, I don't get smooth or round ends.


(sorry if this is really easy to find, I tried looking for it but I only find videos where they change the width of the whole line)

Hi @Techdraconics, the property stroke-width is always valid for the whole path.
So, if you need a thinner part in the middle you need to break the line into 3 pieces and set individual stroke-widths.
Or are you talking about a gradually thinner line in the middle?

Left: 3 paths with different strokes
Right: 2 paths in opposite direction with power stroke (triangle in). I am not sure if you can realize this in one single path.

What about PowerStroke LPE and Pattern along path LPE? Those are meant for this type of thing.
Select path, then Path -> Path Effects ... -> + -> select 'Powerstroke'. Then edit the stroke width with the node tool on the canvas by dragging on the pink diamond-shaped handles. To add more of those handles, Ctrl+Click on one existing handle and drag on it (it duplicates).

Thank you, the power stroke was what I needed.

Awesome tutorial, thanks for this:)