The easiest way to learn how to design furniture is to copy existing designs, then tweak them slightly to better fit your style. After a while, you'll learn how to incorporate different designs into a nearly original piece.

Copy copy copy until you understand what you like, and what works functionally.

It's not that difficult to find plans available either for free or for sale on the internet, though I have yet to build a furniture project straight from plans. I'll usually come up with something I want to build, and I'll look at pictures, watch build videos, maybe find plans, and then I'll design my own version in FreeCAD.

I tend to start in the spreadsheet workbench and list things like stock dimensions, pertinent detail dimensions and such, derive certain dimensions, I like to keep as much math in the spreadsheet as possible, and then I use the sketch > Part Design workflow to create the parts and the A2Plus workbench to assemble them. Creating a 3D model is often a sanity check to make sure all the parts fit together, I sometimes don't bother to model all parts or I'll only do half of a symmetrical assembly. I used to make drawings to print with the TechDraw workbench but for my purposes in the shop I stopped doing this as I would just work from the spreadsheet. If I was sharing my plans with others I would make more careful drawings.

This of course allows you to tailor the work to your space and you.