Oil is simple fill the shock body to the snap ring groove and allow the ressie to get @1" of fluid.
Refill the body and put the shaft assy in , ,have to cycle  it slowly to get most of the air out then fill above the piston so the seal head touches oil before the bleed holes in the side are closed off. Shove the seal head in and replace the snap ring BE sure to seat the seal head on the snap ring. Before moving on. Fill the ressie so oil just bleeds out before the sealing surface meets the body.  I  usually compress the bladder a little so I can use air to seat it. Thers no good place to pull on it without damaging the Schrader valve. You can assemble in the ressi first and the shaft and body second, its a matter of preference no difference in the result.