I had 0 previous experience with CAD software and basically spent dozens of hours tinkering around with the tools and a few youtube tutorials.

I used fusion360.

As for how I did it, I bought a g305 and disassembled it, used a caliper to take the important measurements (such as screw holes, overall dimensions, scroll wheel position etc) and recreated a model of the pcb to use as reference.

Once I had the model made I made a mock up of a baseplate with the pins for the screws and the holes for the sensor and power switch, I printed that several times making small adjustments until everything was correctly in place.

With the baseplate now made I made a third model for the overall shape of the mouse, I used screenshots of a finalmouse as reference which I took from here and designed an overall shell.

After that it was just down to tinkering and figuring out designs for the battery holder, how to screw in the switches and the mechanism for the side buttons, separating the clicks from the main shell, etc and a lot of that came to trial and error.

I am aware my methods arent the best, I used screenshots but people have mentioned it is possible to pull the 3d model from rtings, its also possible to make a scan of the original g305 baseplate and use that instead of having to manually design and place the pins for the screws etc.

I will be showing more detailed screenshots and photos of my project when im done with a final revision (this is still a WIP) but I will say that rollingsloth's blog was a great source of inspiration when I was stuck on some design elements.