Measure from center to center between the two mounting bolt holes on toilet; attachment will fit the length of 5.5”–7.5” between them. The bracket on the attachment can slide to fit the parameters to help ensure a proper fit. A-3 attachment recommended for elongated toilets as long as 18 1/16” from front to back of toilet seat or round toilets as long as 16 1/16” from front to back of the toilet seat; it is not recommended for French curve toilets.
Instructions are pictorial, but even more important, there are pieces missing, namely, washers. When attaching properly, there are numerous leaks at all connections, especially from the "T" to the fill tube. Absolutely no washer, and local hardware stores (major metropolitan area) do not carry, nor "T" (to replace) otherwise, installation is easy. Now, make it workable.