If you take off the round cover on the top of the box, just behind gearbox end the cover. Its held on by a spring clamp. You will see the sliding dog immediately underneath. clicking into second gear the dogs of the sliding boss and the inner spider should line up as closely as possible. If the detent plate on the front of the box is correct and has not come loose then the detent arm should hold it in this position. If the dogs do not line up when you operate the change arm (rotate the dogs to line up first so they can move straight in) then you will need to alter the length of the short connecting rod that joins the footchange cam to the operating arm on the front of the box. This can be a little hit and miss as the two rod ends are usually both RH thread so needing a full turn to line up. If you can get it as close as possible and let the detent ball pull it into line is the best bet.
best of luck