Ian, follow these instructions and you won't go wrong.
1. As everyone says "rough up" both the toe and toe guard with the sandpaper supplied.
2. Apply a very thin layer to both the toe and toe guard. If you apply loads then it will take longer to dry and won't stick as well.
3. Wait around 10 mins
4. Press them both together and hammer (lightly tap) the guard on to the toe to give a good connection.
5. Wait to dry, best leave it overnight for best results (if done right you won't need to add any tape, it will stay in place)
6. Get a good pair of scissors and trim
7. Sand any blemishes smooth.
Good luck