If you do find the encryption key or password, the steps for restoring files and data from the Time Machine backup are laid out in this guidance from Apple: Restore items backed up with Time Machine on Mac - Apple Support
AFAIK, there are no other steps involved other than supplying the encryption password or key whenever prompted. Perhaps another user can provide some detail about this.
There is no way to access the encrypted data on that TM backup without the encryption password.
If you want to avoid this situation in the future, either don't use encryption or don't forget the password or lose the encryption key - write it down.