I didn't quite follow an explanation I saw in another post. I thought the method was to fully load the magazine, put a round in the chamber, and then depress the magazine down, so the bolt can slide across. The other post was describing how to do it on a true mauser, and seemed more complicated...

Not haveing read the other post...
Some extractors will slip over the rim of the cartridge that is already in the chamber. Some will not.
So if it does not, then you can push in on the side of the extractor to force the front of the extractor away from the bolt. Do this while pushing the bolt ahead and the extractor will mostimes slide over the rim and then you can close the bolt.
Hope this helps.

You can close the bolt on the Ruger over a cartridge dropped in the chamber. But:

With a full mag box, you must depress the top cartridge slightly so the bolt rides over it. Most of my Rugers allow me to put the extra round for the chamber on top of the full mag box. I then slightly depress it and the rounds in the box. The bolt face will then slide over the rounds in the mag box while the loose round is depressed just enough to slip under the bottom of the extractor. I let it go and it pops up behind the claw.

GVA

If your extractor arm is to stiff on a true Mauser you can remove the bolt Slide the rim of the cartridge into the extractor grove then reinstall the bolt while holding down pressure on the last round in the magazine. Allowing the extractor captured round to slide over the loaded rounds into the chamber.

It has always seemed simpler to just feed the first one into the chamber, flip it over and fill the mag from the bottom. This has always worked fine for me.

On a Mauser you can pinch in in or push in the extractor with your thumb or birdie shooter and it will go over the case rim and chamber..

You can also very easily convert the extractor to ride over the rim with a bit of metal removal, but I never do this as it probably weakens the extractor, and that strong Mauser extractor is one of Mausers best attributes IMO.