Install TeraOS.

One file to download. Put it on a USB stick for a PC, or hand it to a virtual machine. The installer asks three questions and does the rest.

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A PC that's not too old. Most laptops and desktops from the last ten years, with a 64‑bit Intel or AMD chip. It must start in UEFI mode with Secure Boot off, which is a setting in its start-up screen.
A USB stick of 4 GB or more. Everything on it is erased.
A disk to install to, 32 GB or more. Everything on it is erased too. The installer shows each disk and what's on it, and asks before it touches one.

Works in VMware Workstation or Fusion, VirtualBox, UTM and QEMU.
Make a machine for Linux, other 64‑bit, with UEFI and Secure Boot off, 2 processors, 4 GB of memory, and a new disk of 40 GB.
Put the file you downloaded in its CD drive and start it.

On a PC, step by step.

Write the file to the stick. Use balenaEtcher (Windows, Mac, Linux) or Rufus (Windows): open it, choose the file you downloaded, choose the stick, and press the button. It takes a few minutes.
Start the computer from the stick. Plug it in, turn the computer on, and pick the stick from the start-up menu (usually F12, F2 or Esc as it starts; the first screen tells you which).
Answer the installer. Your keyboard, your time zone, and which disk. Then it installs, in a minute or two. No internet needed.
Take the stick out and restart. TeraOS asks your name and a password. The password locks your files, and it's what you type to get in.

There's no app store or file manager yet, and things will change. Try it on a spare machine or a virtual one.

Your files are locked with it. There's no way back from a forgotten one yet, so pick one you'll remember.

TeraOS checks once a day and asks before downloading. An update takes effect at the next restart, and one that fails is undone on its own.