Hello
@timc3,
Yes please follow the above guide, which lists the preparation steps or prerequisites. When the prep work is done, then...
...follow
this guide to install OpenCore 0.7.0 (yes, it's good to start with 0.7.0). When you open that guide, click the link shown below to install OpenCore 0.7.0 using the HackinDROM App.
View attachment 528239
After the OpenCore EFI has been created and installed in the EFI partition, boot your existing Mojave disk (with OpenCore) to make sure it still boots.
If everything is okay, then perform in-place upgrade to Big Sur.
After reviewing these guides, if you have any questions please ask.