I can tell you how I did it. Thats not quite the same as telling you how to do it.
I won't be there wth you and I have no control over what you do.
You are on your own. You assume all risk. The following is not a set of instructions. I'm telling you what I did.
Virgin brass has not been work hardened. The necks are less likely to split.
I use Cream of Wheat. Nothing else will do. Not corn meal.
I use Bullseye. Its very quick. Quick pressure is needed.
NO BULLET IS USED!! Using a bullet will likely blow something up.
I use enough loading blocks to hold all the rounds vertical.
I use a powder funnel to fill the cases.
I made a powder dipper for the Bullseye.
Now some background. If we were loading ammunition to shoot bullets ,we woud be quite careful about charge weight. We would want to control it to 0.2 tenths of a grain or so.
This is a little different. Only Cream of Wheat will create the back pressure.
Some place,maybe 30 years ago,I read a guideline on "How much?" Bullseye to use.
The sugestion was to start with about 10% of a normal charge for that cartridge. Example= 50 gr or so for normal 30-06 loads with typical rifle powder for 30-06.
So 10% would be 5 grains. Thats ok for a start,but I did not get complete results. I gradually incrased the Bullseye (and my dipper) till I got good looking 35 Whelen brass. That was a long time ago,but my memory says 7 grains. Fwiw,I made my dippers out of hard maple and drilled the cavities. I could hand turn the drill bit to increase the dipper capacity till I got results.
I will say if I did not get good results by the time I reached 20%.or 10 gr of Bullseye,I would stop. Explore another method,which would be a tapered arbor die.
This decision was not based on any science ,testing,or measurement. I cannot verify it. Its the "Boundaries of caution and pucker factor" Thats what I did.
OK. I primed my virgin brass. I put one dipper of Bullseye in the case, and then filled the case till up in the neck with Cream of Wheat using a powder funnel.. Some methods are less messy than others. I think I made another dipper. The bench powder measure I use to dispense powder might work. I did not do that.
The end goal is a case full of Cream of Wheat sitting on top of a small charge of Bullseye. The loading block and bench covered in C.O.W mess is not the goal.
If I had loaded 100 at 5 grains of Bullseye, they would not have fully formed and I'd have a mess. I increased the charge gradually till I had a charge weight that worked. That figured out,I loaded a bunch.
Then I took the full loading blocks to a safe place to fire them.
I did not have to worry about bullets,but explaining it all to Law Enforcement could be an awkward moment. I chose a place where the sound of gunfire does not result in law enforcement response.
OK Its an option to put a little ball of toilet paper as a stopper in the neck. It helps prevent the C.O.W. from escaping.
I was doing this in a 1903A3 Springfield action. Obviously,filling the magazine was not going to work. Forcing the controlled round feed hook extractor to snap over was not the best idea.
So,the gymnastics amounted to pulling the bolt,snapping the cartridge in the bolt face,and,muzzle up,re-inserting the bolt in the rifle.
All that done,go bang,and inspect your nice piece of 35 Whelen brass.
Now, this method gave me beautiful concentric necks and shoulders,and uniform neck wall thickness. Just jamming 30-06 brass into a 35 Whelen sizing die resulted in spindle deflection, case necks thin on one side,and odd,off center shoulders.
But fire forming wth C.O.W. IS a pain in the butt.
Later,to make 40-70 Sharps Brass out of 30-40 by the C.O.W. process (blowing it out straightwall) I turned up an insert out of an old take off barrel to have the outside dimnsions of a 12 ga shell,then chambered the insert with the 40-70 reamer I made.
It was a lot easier to use that chamber insert with an old singleshot shotgun to fire form the ammo.
I did all this and got good results. It was a pain in the butt. I have done my best to answer your question without giving a lot of advice about "What you should do"
A brother built some AR rifle where he is necking up 6mm Benchrest brass to 6.5. I do not know which brand it is,but some reloading die company makes a die with a rigid tapered expander spindle...or maybe he just found a spindle/arbor to buy.
He is getting excellent results.
Good luck!! I liked my 35 Whelen. It was fun to shoot.