Responsive notifications page made with Flexbox

Overall, as a beginner, it was really fun doing this small project, but I kind of found some things very difficult to construct and I had to pay great attention to all kinds of details.

My question is, how can I create the best strategy for constructing a site without having to deal with these brain-frying problems that come out of nowhere? Because I'm sure that somebody who had some type of plan and experience could do this page in 30mins, whereas I finished it in 4 days.

Any tips are welcome! I will gladly accept any kind of code corrections as it helps improve my skills further.

Thank you very much in advance!