Pull the fuel rail off, install 1 injector at a time, with some rubber grease on the o-ring,
apply pressure and twist the injector at the same time, have the injector on a 20ish degree angle, so the o-ring sits into the rail more in one area, until the o-ring is fully in the rail then straighten it up,
put all 6 injectors into the rail, then fit it to the manifold. been careful not to have a injector come out.
PS take your time, don't rush it.
Nigel