The best "apprenticeship" is some years of just single or multi-day winter camping and mountaineering trips. When I first started with this stuff I did lots of solo winter trips to the Daks and stayed at Marcy Dam in the lean to's for days at a time and roamed through the high peaks summiting most of them in winter eventually. The idea is just to get comfortable with being outside and in the cold for extended periods.
I have to say it was ultimately the most rewarding thing I could have done to build up this kind of experience slowly, all by myself. I never felt like I needed to be guided up anything that I wanted to climb.
After a few years of this I was ready for the Presidential Traverse, which I did with a group of similarly experienced friends.
Especially in a small group everyone should be strong and experienced and you all have to watch out for each other.
You also have to gain as much experience as possible judging weather conditions. As someone else said, you don't want to be out there when the weather is really bad. This kind of Mountaineering or any serious Mountaineering is all about waiting for the right conditions and even then you have to be prepared for things to change- the mountains are unpredictable sometimes.
My best partner in the mountains was literally someone who grew up in the mountains- his father was a ranger in the Daks. People like this develop a "sixth sense" about the mountains which is invaluable. Reinhold Messner also writes about this. If you know when to retreat, you will be much more likely to survive in the mountains. So many accidents you read about could have prevented only if... the "unlucky" victims had known when to turn back.
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