Log in to Admin area
Go to Design > Layouts section
Open main design. Often called top memu design or left menu design, etc. will have default tag and be type of Layout (bldlayout).
Click Duplicate from three sot menu next to save button to make a copy of the main design.
Adjust the name (title) of the copy
Remove the default tag
Change the type to match the type of item you want the design for (usually Page).
Change Layout to No Layout
Adjust design elements in page editor.
Save and Go Live
Open page ( or other item) that requires a unique design
Change Layout and select the newly created design.
Save Page and Go Live