The implementation command is already mentioned in lammps manual.
Fix 1 all bond/create 1 3 5 0.8 1 iparam 2 3
This means that you want to create a new bond between atom_type 3 and atom_type 5 every step when two atoms within 0.8 (unit) distance. Then a bond type will be made as 1. Atom type 3 will be created two bonds on it (e.g., iparam keyword).
Note that you should define your atom types clearly to make a new bond with this command.