Migrating a Mastodon server out of Masto.host can easily be achieved if you are using your own domain/subdomain. As mentioned on what you need to start your Mastodon server on Masto.host that is not the case if you are using a free Masto.host subdomain.
- plan in advance because there will be downtime
- make a Mastodon clean installation
- the day before, lower your domain DNS TTL (recommended for faster DNS propagation but not necessary)
- access your my.masto.host account
- on the hosting details page, click 'Download Backup'
- click 'stop your server' under 'generate a backup from live data' (you should not restart the server on Masto.host to prevent federation content/activity from being lost)
- choose the data you want to download. To make the migration simpler Media Files should include remote cache
- wait for the download link email (it can take hours depending on the amount of data on your server)
- download the data
- you have everything you need to follow the Mastodon guide about migrating your server
Feel free to reach out firstname.lastname@example.org if you have any questions.