ssh into the printer (putty/MobaXTerm/command line/powershell/terminal).
You'll need to identify the IP address of the printer; I recommend AngryIPScanner for this job, found here: https://angryip.org/
Sort by hostnames after you've scanned. Then ssh in with user: root, password: makerbase
Run these commands... (when NOT printing!!!)
sudo apt install git
cd /home/mks
git clone https://github.com/mainsail-crew/mainsail.git
chown www-data:www-data mainsail
chmod 0644 mainsail
Install WinSCP from here: https://winscp.net/eng/download.php
When it runs, choose SFTP and specify your IP address, username (root) and password (makerbase)
Copy the mainsail file from this repository into /etc/nginx/sites-available
Run these commands in your ssh terminal
ln -s /etc/nginx/sites-available/mainsail /etc/nginx/sites-enabled/
systemctl restart nginx
If any command complains (it shouldn't since you're root), then rerun it with sudo as a prefix with a space e.g.:
sudo ln -s /etc/nginx/sites-available/mainsail /etc/nginx/sites-enabled/
Now run
systemctl restart moonraker
systemctl restart klipper
Now Mainsail should be available from here: http://(your IP address without brackets):8081
TADA!