The way I did it was to make a rectangle the size of the area I wanted planed, using inkscape, then export it as dxf. Load it in estlcam and use the inside cut tool and then in the pop up window choose pocket. Then set your depth you want and how much to take off on each pass. Just don’t do it like I did or it will take all day. I put 1mm total to come off and I removed .25mm each pass and it ended up taking like 5 hours. I could have done it all in one pass but I had never cut anything before so I didn’t know yet how much I could take off. The dw660 is a little beast and can handle more than that. The end result though is a perfectly flat cutting surface.