Create your channel under the team that Mio Universal Channels is installed on. This is the same as creating a normal Microsoft Teams channel.
Fill out the desired fields.
For the Channel name, name that channel the name you want the Slack people to see - it can be the same as the Team name.
Make sure Privacy is not private (The Universal Channels app will not work on private channels, only public ones). Once done, click "Add"
At the bottom your channel, find the "· · ·" and select Mio Universal Channels.
Choose the first option, Convert and invite.
Congratulations! Your Microsoft Teams channel is now a universal channel.
If you're having trouble seeing the window, you may want to check if the Microsoft Teams app was correctly installed to your team first.
Will external contacts in different channels (but in my same Microsoft Teams team) see each other's messages?
Nope. If you have separate channels for different customers, but you created them all under one Microsoft Teams team, they will not be able to read each other's message whatsoever.
External contacts will only have access to the channel that you invite them to, not the team.