How to keep projects under version control with Git?

I’m really curious about this so would be happy if someone could provide me with a solution to this problem.

Scenario:

Assume I’ve bought a plugin from Envato Market. The plugin has a version of ‘1.3’ but I found a few bugs in this script so I decided to put it under version control and fix these bugs myself. My question is, what should I do if the author releases a new version of his plugin 6 moths later (1.4) with some additional functionalities and I want these changes in my version of the code too?

I’m thinking I should put the author’s code under version control, and create a new branch for each version he releases so I can keep track of changes and put the code into my own branch. E.g. ‘1.4_mine’.

Am I thinking in the right lines here, or are there any solutions to this? An automated solution would be awesome but I don’t really know what I’m looking for so that’s why I’ve asked here. As code is downloaded from Envato Market in zip format I’m a little confused of the best possible way of keep track of changes to the code.

Thanks!