Download and extract the latest version of Mainsail from their Github

Open the Raspberry Pi Imager

Select Operating System > Use custom > Select the Mainsail OS you downloaded.

Select Storage > Select your SD.

Bring up the Advanced Settings menu by pressing the settings cog.

Here you can setup your WiFi, locale, enable SSH and numerous other options.

Write > Yes to confirm

After imaging is complete the SD is automatically ejected. Insert it into the RPI and power it on.