I don't think there is any value learning how to play base game mistfall first. The rules are better in heart of mists.
How about this as a suggestion?
Put the 'time' cards from original mistfall aside.
Put the original mistfall rules aside.
Put the original mistfall player marker aside.
Mix everything else together: location tiles, enemy cards, encounter cards, reward cards and use heroes from both games.
Use the rules from Heart of the Mists.
Go to the files section on bgg for heart of Mists, Download/print the 'Learn to play Heart of mists'
Do the original Mistfall quests first.
Then the Heart of mists quests.
Use any heroes you want.
This is the best way to start, I think