In TSRWS, go to Misc tab, you can see wall mask images there.
In many cases, your own mesh and EA clone have different shapes, so you have to edit wall masks to fit to your mesh.
Click "Edit", and click "..." button.
Then you can see new tool bar on the top of preview window.
■ = Add rectangular cutout, □ = Remove rectangular cutout, ● = Add circular cutout, and ○ = Remove circular cutout.
First, remove the original EA's cutout using □ or ○.
In this example, I drew a rectangle to cover whole area of the original mask.
The cutout was removed. Next, you have to add cutout fitting to your mesh's shape.
Select ■, and drag mouse to draw a rectangle along with your mesh.
Once you get a cutout shape you intended, click "Close" button. (I recommend to save the working file.)
Also, you can have 2-tiles or more cutout for an arch functioning as 1-tile arch.
In the cutout edit mode, draw the shape you like.
Then, TSRWS automatically adds necessary wall masks. (Please ignore the weird mask shape in the above example.) Edit the mask as you like.
That's it. :)