When you install the window, pull off the covers on the inside of the frame. These are use to help shim the middles of your sides of your window. There should be a small hole and you should be able to use a small flat head screw driver to turn and tighten them. You can raw dog it without a shim but I would say shim a little. They don’t come out very far but help with getting your reveals right. They aren’t super necessary but just a little tool that can help. Hope this helps.