Step-by-Step

This is a Tutorial explaining how to do it in 10 simple steps..

This is exactly what I did, and both of my Macs are running well.
I just recommend having at least 8gb of RAM. My MacBook Air with 4gb is suffering a little bit.

Check if your model is supported by OpenCore.

Let's Make It Happen:

One: Download from this link the Installer.

Two: Download OpenCore Legacy here.

Three: Extract the installer into your Applications folder.

Four: Then, right-click into it after the process is done.

Five: After right-clicking on "Show Package Contents", you will find The InstallAssistant inside the Resources Folder.

Six: Open Terminal and write sudo

Seven: Press Space and drag the InstallAssistant into the terminal window.

Eight: Write: --volume and Press Space again

Nine: Drag the Usb (16gb Minimum) into the Terminal window.

Ten: Press Enter.

Voila, you have the USB installer!

Now,

Install OpenCore TUI into another secondary Usb device.

When you boot, press the option key and choose the EFI option, from OpenCore. This way you will trick the installer and it will work on your MAC.

Air:

If you have any problems booting your MAC after Monterey, just install OpenCore Legacy Patcher on your SSD. I solved this problem on my MacBook Air.

And remember kids, always back up your data.

Pro:

Any question?
Just ask in the comments.

Best Regards,

@xmauron3