The process of installing the windows in the tiny house is becoming a trying event. The house is not perfectly square, but the windows are! This means that one side of the window is a bit loose in the frame while the other side won't even fit.
The large window, which requires two people to manage, is test fitted into the hole where problem areas are identified. The usual culprit is the plywood sheathing. When I cut the window holes out, I wandered a little from a perfect straight line. These wandering a lead to small peaks in the plywood projecting into the window frame.
The window will rest on this small protrusion and rock to one side or the other, making one of the corners overlap with the wall and not fit into the frame. To trim off these small annoyances, I use a hand saw and cut flush with the 2x4 framing. This removes the error while providing a clean smooth surface to receive the window.
After several test fits, the window will finally fit into the frame! A few screws are placed on each side of the window to tack it to the wall and avoid any disastrous window jettisons. This allows me to walk away and observe the window from a distance, to evaluate the squareness from a far. If the window seems crooked, now is the time to correct it by placing small shims inside between the window and the frame.
Shimming the window is temporary, as it is only needed until all the screws are installed. Shimming also gives you the ability to hold the window in place while you re-evaluate its squareness. Building on a trailer not set on level ground precludes you from using a plumb bob or bubble level, instead you need to rely on your eyes. Comparing the sides of the window with the sides of the tiny house will tell you if it looks straight or not. In the end, looking straight is the whole objective!
With the window straightened out, all the screws are installed, wrapping the window in a barrage of tiny stud screws which collectively hold this massive window in place. Once all the screws are installed, it is safe to remove the shims as they are no longer needed nor functional in this type of construction.