The holes on the battery for the battery mounting screws may not line up exactly with the threaded receptacles on the MacBook chassis. You may have to exert some pressure on end of the battery to get the holes to line up. Recommend installing the longer screw (adjacent to the memory chip) first.
Excellent instructions! May want to add that until the replaced battery has charged, the operating system may throttle the CPU speed. Among other things, this can lengthen the time to boot considerably.