There are two (2) methods for finishing.
1. De-grease, blast the part to be phosphated, rinse in water, imerse in the pre-black, rise, imerse in phosphate solution (20 imn.), them imerse in boiling water.. For just the receiver I would remove it from the barrel assembly, de-grease. blast, pre-black, phosphate, then imerse in boiling water. After that I would re-install the barrel assembly being very careful not to damage the finish on the receiver.
Below is a shotgun that I did just parts using the pre-black. The gray receiver and barrel assembly were just phosphated (no pre-black).
2. De-grease, blast the entire barreled receiver assembly, rinse in water, imerse the barreled receiver in pre-black, rinse in water, imerse in the phospahte solution (20 min), rinse in boiling water. Do this to each metal part of the rifle (if you wish) so that all parts are the same finish.
Below is an Enfield that had most of the finish worn off (the rifling was beautiful), so I used method #2 and pre-blacked the complete rifle. Then added a new stock....it's like you have a new rifle.
As a warning, phosphate may weaken springs and cause them to break if phosphated. I routinely remove all springs prior to blasting and phosphating. The big problem is the AK's mag catch spring. I haven't phosphated an AK mag catch spring to see if this is true. Others have accomplished phosphating without any problems to the spring. I just added this to let you know that it can happen (it came with the phosphate instuctions I use from a long time ago).