You need to make sure the window frame is plumb, square and level. This is easiest accomplished by mounting through the sides. It is possible to mount top and bottom and still have a nice finish. You will need to use shims to "set" the window in it's final position. Then use the screws to mount it up. After the screws are in place, recheck for plumb, level and square AND raise and shut, lock the window. Mix some stiff mortar mix and stuff it in close to, but not against the shims, after the mortar cures, remove the shims and fill in the voids where the shims were.