I've done some initial internet searches, which suggest I just root prune now, and let them grow a year or two in pots before beginning to trim them up. That's good advice. What I would do is:
Be very careful not to let the roots dry out. It only takes a few minutes to for fine roots, the ones that do all the plant's heavy lifting, to dry out and die back.
Trim the long roots back a bit to stimulate root branching. When you pot the trees up or put them in the ground, arrange the roots over a cone of soil to they spread horizontally. The appearance of strong rootage is particularly important to the illusion of great age bonsai practitioners seek to convey.
Allow some branches to grow on the lower trunk as sacrifice branches. These will serve to thicken the trunk, add taper to the tree, and won't be a part of your composition when it gets to the refinement stages.
The image below has only a single sacrifice branch remaining, it's only purpose is to thicken the trunk. It is intentionally positioned so when it's removed the temporary scar will be hidden on the back side of the tree.
Rub off the buds that occur where branches attach to the trunk or the trunk bifurcates (splits). Too many branches close together will cause conspicuous localized thickening of trunk and branches. This is called reverse taper and should be avoided as it can wreck a tree's suitability as a bonsai candidate or force you to change to a different course that allows you to eliminate the reverse taper.
This air-layered hackberry has moderate reverse taper near the top of the tree. I realized it before I layered it and I have already planned to correct the problem by removing it and using a branch from below the swelling as part of the tree's eventual top.
Others have suggested to buy "pre-trimmed," or they want to sell you "bonsai-ready" redwoods at premium prices. You'll find that there is no such thing as bonsai-ready trees. Most trees sold as pre-bonsai have been roughly sheared to keep them small instead of being pruned, which usually results in serious flaws you'll need to eliminate or work around, something that can set the tree back far enough that you might have been better served to grow your own trees. I can't stress enough how happy you'll be (if you become a life-long bonsai practitioner) if you put trees in the ground now and tend to them with the idea they'll be ready to be worked on 5-10 years hence. This will afford you the opportunity to learn how to grow trees on as you advance in the art/science, and allow you access to trees you'd otherwise pay hundreds of dollars for because of their age and the work you've done.
I've also heard redwoods are easy subjects to begin with, but hard to make look great as a bonsai subject for a beginner. I read this as- "You won't kill it, but you also won't show it to anyone as a sample of your abilities." Vigor is a genetic trait, and DR is a very vigorous tree, which means no more than it's better than most trees at tolerating stress/strain. IOW, the tree is tolerant of a wider range of conditions than most other trees. As such, it's a good candidate. Some people take to bonsai like a bird to the air. Others struggle with either the artistic half or the science of keeping the trees in a state of vitality that allows their trees to tolerate the frequent work we're so anxious to undertake. If you have enough trees, you quickly pass through the stage where you want to always be working your trees, and that's a good thing. Learning to put vitality of the tree at the forefront will save a lot of trees from over-working. Your first efforts might be really special, or they might not be. It depends on your eye, your ability to see how fine bonsai specimens are put together, and how quickly you pick up on the basic techniques.
Bonsai is pretty close to letting a tree grow with or without guidance, then finding the tree within the tree. Sometimes that takes leaving 90% of the tree lying at your feet. Once you have discovered the tree you want to create, you set about its creation with a plan that is likely to be refined several times along the tree's path to becoming a bonsai. The skills you will acquire as a bonsai practitioner will be able to be widely applied to virtually every plant you grow, whether it's pruned or not or in the ground or a pot. Plants are plants, and as a bonsai practitioner you'll be growing under much more difficult conditions than the average gardener or container gardener. Like a high diver, your skills will need to appreciate in order to deal with the difficulty factor you're saddled with. The personal rewards can be immense. I'm seldom more relaxed than when I'm working trees and can stand for hours pruning and repotting with virtually no sense of the passage of time or that I might be working in near darkness because it's 10 0'clock in the evening. I hope you get to that point.
Don't be discouraged if your first efforts aren't as fruitful as you hoped. It's not something you just decide to do and do it. It will take some dedication because your trees are more like pets than plants. You'll learn the individual personalities and quirks of individual trees, even of the same species.
Learn what a trunk chop is. It's used to promote the rapid taper of the trunk. Chopped trees:
Al
