Something like this:
(Where is is thicker/thinner in certain parts)

create a rectangle and then add nodes and use node modes to form them into those shapes

You could use the pen (bezier) or pencil (freehand) tool with a 'Shape' other then 'None': see the manual section about the Bezier (Pen) Tool for details (scroll down to "Shape: Applies a shape to the path after it is drawn").

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I'm not sure if ~suv is referring to spiro splines, but if not, that's another way to go. Also you could use the Calligraphy tool. And especially if you use a graphics tablet with pressure sensitivity, makes it really easy to do.

brynn wrote:I'm not sure if ~suv is referring to spiro splines, (…)

No, I was not referring to spiro splines: the usage of a shape with the pen/pencil tool (Pattern along Path LPE) is not bound or limited to drawing those spiro splines swirls.