How would I go about creating an alternative frontend for a website?

Ok, so I've been thinking for a bit, and I know there's a ton of alternative "libre" frontends for websites like YouTube and Reddit. However, I was thinking: what If I just wanted to make my own frontend? Not specifically for the intentions of "libre", but I wanted to change the controls and appearence of the website. I'm honestly unsure of how to approach this. Any thoughts?