I am making something like a frontpanel off plywood. The panel will have some round and rectangle openings/holes for buttons and displays.
I'll prepare the template on paper then stick it on the wood and do the cuttings following the template.
I know I could do that simply with a pencil and a ruler, however I would like to start learning to use a simple design/CAD software. Have you got any suggestion?
Ideally I'd like to learn a software that I could use it also for 3D modelling (as I'd like to get into 3D printing at some point soon). And if it is usable online (no installation) and free even better!
Libre Office Draw. Libre Office is freeware, already installed on Ubuntu and Raspberry Pi. It is available for that other operating system too. I draw modules with the mounting holes marked with a +. The holes come out exactly right if you measure right and use the internal ruler.
I recommend the free open source Inkscape for 2D designs:
As for 3D, I did evaluated the available free open source options some years back and settled on FreeCAD, but I don't have much experience in this area and it's likely new options worth looking at have appeared since that time. OpenSCAD was also intriguing to me, but I decided that I don't do enough of this stuff for it to be worth while to learn a dedicated programming language.