Open the PostmarketOS Downloads page, and find your device in the device table. You will find options for your device's Stable version, and an option for the Edge version.

Edge will have experimental features, but might not be stable, and might make daily driving difficult.

Once you've found your device, and have selected which image you will be using, you will be brought to a directory page, where you will be prompted to select a user interface.

After selecting your user interface you will be presented with multiple files to download.

The first file is your rootfs, and will have your main system partition. This file will be {codename}.img.xz.

Depending on the device, you might need to download either the Android Boot Partition, which is the {codename}-boot.img.xz file, or the split boot/root layout file which will be named {codename}-bootpart.img.xz, in addition to the main system partition. This isn't always the case for every device, refer to the device's wiki page.