0. pick text editor 1. install text editor 2. find some sample html 3. open file in your browser 4. find some resources for learning HTML & CSS (yep, w3schools) 5. learn that validators exist 6. learn that linters exist etc. etc.
With mozilla thimble: 1. click to start new project 2. try stuff (instant feedback), learn about code validation (instant feedback), share your page etc. 3. level up to more robust environment.