" Please be detailed,"
Asking an old f@r! to be 'detailed' is not a good thing
The weapon is (factory) new or new to me. Before I trade or buy, I will have studied the subject, a lot. So I will have a good idea what I'm getting into.
The weapon:
Strip it down as far as prudent. Clean/scrub/burnish everything. This may be the time to clean the barrel. That process is to patch the barrel (breach to muzzle - if the breach is blocked/barrel set in the receiver, think M1/M14, insert the cleaning rod from the muzzle, patch and pull) with tight patches until clean and dry. Plug the chamber throat (cotton balls work well) and from the muzzle run a loose patch soaked with Butch's Bore cleaner, plug the muzzle and let it set. (I do this outside on my pick nick table, that stuff stinks!) After a bit, pull the plugs and run patches until no green is on the patches, repeat the process until no green is on the first patch. On older/well used weapons, I sometimes will run a nylon brush down the bore during this process. Finish up with some Hoppies to kill the stink.
Put it back together and oil/grease as needed. Special care taken with the actions lugs and fit into the stock. If it doesn't look like a very solid fit, contemplate 'glass bedding'. Moving on. Is the stock in contact with the barrel? Is it supposed to be? Deal with it as needed. Seal wooden stocks before continuing. With all screws/bolts firm to tight, look at the sighting system. This is the time for any improvements. Assuming the sites are OK and tight, the weapon can be considered ready.
Ammunition prep:
Obtain/find or select a quantity of brass in or from the same lot.
Tumble/polish as needed. I want to see any imperfections in the brass.
Inspect and sort. As I do this, I check the web for beginnings of head separation (I seldom buy NEW brass).
Lub and full length size. De-prime by other means if the brass has crimped in primers first and remove the crimp ring too.
Dump the brass into the tumbler with steel pins, Dawn soap and water. Tumble over night, rinse with clean water and dry in oven (preheat the oven to 250 to 300 degrees, put the brass on a towel on a cookie sheet, turn off the oven and put the cookie sheet/s in the oven. When the oven is cool, the brass is dry.)
Trim all brass to length, inspecting each casing. De-bur in and out.
Hand prime all cases.
I have not discussed annealing at this point. That is another complete subject.
We are getting there.
The Loads:
From a known and trusted source, pick a starting load for the selected bullet weight/type and powder. (I tend to use 748/IMR4895/H4895/Varget/4831 powders. The load is the determine factor.)
Over all length determined by several factors. Does the bullet have a canalure? Type of magazine/feed system. Looks. I like/want a minimum of one bullet diameter of bullet in the case neck.
Load 3 to 5 rounds. (As my range is just outside my garage door, I load and test, one sample at a time.) Log all information. Test fire these loads over the chronograph, record everything. Look over each fired case, primer condition, soot, whatever. Log it if note worthy. If being fired in an auto-loader, does this load fully function the weapon? If not, what didn't work? Was the measured velocities what was expected?
Repeat this process with increasing charges of powder until velocities/recoil/ANY pressure signs are beyond what is desired.
Analyze the collected data. I put it on a spread sheet. Pick a loading block that gives the most desirable results. Consistency in velocity and smoothness of functions.
Move on to 'fine tuning' the load for optimal accuracy. I normally start shooting at 25 yards. (I can walk to my target stand.) This gives a quicker 'zeroing' of the sights and general idea about how well or poorly the finished load may be. Move the target out to 100 yards. (Now I have to used my 4wheeler, it is not a direct 100 yards to get to the 100 yard target stand, out, around over the bridge, up the hill and over and finally down to the target. A simple straight visual view with maybe 300 yards of ground travel to get there!)
Differing weapons/calibers will have very different potentials. I had a 7.7 Jap, late war production rifle that I was happy to get 3 shots inside of a 4 foot circle. The rear sight was a 'peep'' that was welded to the action! Had a single heat treat 03 barreled action that would put 173 grain gas checked lead round nose in two inches shot after shot. Rainbow, 1500 to 1650 fps. much slower and they lost stability. Didn't trust the steel for any more velocity, lead loads or not.
Don't expect to shoot as well with that .460 Weatherby loaded with full power loads of 510gr full patch bullets as you can with that .222 Rem X40 and 45 grain HPs.
Nuf babbling. Told you, never give an old f@r! the go-a-head to babble.