That's awesome you are getting into Method now and starting to design your own screens!
In terms of previewing, from the 'Screens' tab link, under 'Customize', you can click the 'View' link per screen. The 'View' link allows you to preview that particular screen. Now if you are actually editing the screen, the only way to see what it is going to look like is by clicking on "Save & Close" and then click on 'View'.