Before creating a website, it is a good habit to draw out the basic format of the website. Things like where should be the location of the title, shortcuts and where the page information should be changed. this would be good for the html and the css part. Knowing how the website display is going to end up before starting to create an website helps very much.
It would be a good habit to write down these ideas in a paper, like brainstorming and with this it is easy to see what has to be done first.
So with this todolist example, first we would decide where the input and button box to be located. then we should decide where the todolist should be rendered. With CSS, how the display should be designed. And then we can create display function(render) first. And make a function to add the todos. Then make the function to checkout the todos that are finished and then make the function to delete the function that is checked out.