So I want to print a holder for some supercaps.  Anybody here disign in 3d regularly?

I use tinkercad from time to time to design some simple things.  But I’m not an expert.

So I want to make a simple box and then add a square hole in to that's 3 mm smaller all the way around.  I can't figure how to align those objects, group them and then center the object on the table.  Any thoughts?

Have you got the way to align them?  Actually it's very easy.  You select the objects you want to align.  Then you click on the align button on top of the screen.   You'll then see the black dots with lines on your parts.  Click on those dots and your parts will be align accordingly.