There are a few different options for flashing your SD card, but one of the easiest options is going to be using a software like BalendaEtcher.

In Etcher, click "Flash from file" and select the PostmarketOS image you downloaded earlier.

Select the USB flash drive as the target device.

Double-check to ensure you have selected the correct drive, as the process will erase all data on the selected drive.

Click "Flash!" to start the process. Etcher will copy the PostmarketOS to the SD card.

This option is best for devices that boot directly from an SD card. PostmarketOS will resize the newly-added partition to fit the entire SD card on first boot.