Don't be shy to ask from the owners on where to start and areas that might be affected by the desired changes. Contributing to Open-Source is also a great way of improving yourself by learning from others. Cheers to contributing!
From there - little by little you will surely be able to work your way in completing the task. And, add a tad more confidence. You are doing great!
This Chrome Extension is a must for any developer - and they are Open-Source too (which I contribute as well). Guess what? They are using Vue!