If you change the nipple,make sure the threaded length,is not too long as screwing it all the way in,
will create interference with the drive,affecting your speedometer reading.
I use a Lincoln #5803
Push the spring loaded ball in, and grease.
Since you need 3 hands-
I leave the gun on the ground,hold it with my knee,hold the needle,square and firm to the nipple,
If there isn't any ancient petrified grease in there,
a pump, or maybe two, should suffice.
You should see a radial pattern of grease,after the first ride.