I’m wanting to cut out a rectangle say 5 by 8 with 90 degree corners but on the inside of this is a nice design that I would like rounded edges is there a way to do this and if so how thanks
I think you would be able to do what you want (from your description) by using Easel. You can use multiple shapes to create a design. Your outside rectangle with 90 degree corners can be one shape. Your inside rectangle with rounded corners can be another shape. Then your design can be other shapes inside of the rectangle with the rounded corners.
Click on the support tab at the top of the Inventables website. Within that tab you should see a link to Easel tutorials. Select the tutorials that demonstrate how to work with different shapes.
Hope that helps.