Step 5: Assembly
Turn the desktops face down on the floor, diagonal cuts together. Place the two-piece latches, closed, on the joint and mark and drill pilot holes for their screws. Be careful not to drill through the top side. Attach the latches. Unhook them and separate the pieces.
Place the corner support in the corner and the file cabinets to either side of the corner in their approximate positions. Place each desktop half, top side up, where it's supposed to go supported by the file cabinets and corner support. Latch the two halves together with the latches you just installed above. Hopefully both halves will move as one now.
Line up the file cabinets along each wall, maybe moving them 1-2 inches out from the wall to allow cords and such to go behind them. Put them under the fattest part of the outer curves and mark the undersides along the sides of the cabinets. Unlatch the desktop halves and turn them over. Align the pieces of quarter round along those marks, flat sides toward the underside of the desktops and toward the filing cabinets. Make sure they're short enough to not be seen when the desk is right-side-up. Pilot drill and screw them down with 1" screws.
Place the desktops back up on the cabinets where they belong and latch them together. You most likely need a little more support in the middle right under your arms since the latches are the only things holding the top together and they occasionally become uneven. I used a corner bracket screwed into each side. In elegant perhaps, but each half is level with the other and there is plenty of support for my hands, keyboard, forearms, yada yada yada.