It's usually easier to put the injectors into the fuel rial first and then put the assembly into the intake manifold. .
Agree, just did this couple years ago. Use K-Y or such and install in the rail first, watch closly that the O rings don't pinch when going into the rail.
Choose your side, but install all the manifold side O rings in the manifold on one side, and install the other side manifold O rings on the injectors.
Now start the injectors without man O rings into the man and then tilt down the other side