You go to https://web.telegram.org. It can also be done with the windows app but I haven’t tried it with the android or IOS app.
Then go here
Create a new group
Now it offers you your whole list of Contacts. Click on two (as a minimum) and go to Next.
Enter the group name and Create it
Then click here
and upgrade it to a Supergroup
and confirm the resulting message.
Now you need to get the link which others can use to join it. Click again here
and you get this
Right-click on the URL and copy/paste as required
