First, set the Stock to the correct size as described in:
https://carbide3d.com/hub/courses/create/job-setup/
Then draw an 8" x 8" square as described in:
https://carbide3d.com/hub/courses/create/shapes/
and
https://carbide3d.com/hub/courses/create/object-manipulation/
Then assign a toolpath as described in:
https://carbide3d.com/hub/courses/create/toolpaths/
Use Current Selection
Select a suitable tool as described at:
https://carbide3d.com/hub/courses/create/tool-library/
(note that the size of the tool will determine the radius at the corners)
and verify it in 3D as shown at:
https://carbide3d.com/hub/courses/create/simulation/
Setting the zero relative to the stock is as shown in:
https://carbide3d.com/hub/courses/running-shapeoko/movements-zeroing/
which should have been covered at:
https://carbide3d.com/hub/courses/running-shapeoko/hello-world/
c.f.,
I would recommend working through a couple of the projects at:
https://my.carbide3d.com/#Projects
before striking out on your own.