Ask HN: How to prepare for take-home assignments?

I recently had an interview and completely flubbed the take-home assignment. I wonder if anybody else has good repository of take home-assignments for practice, or some good guide for training.

This is not a complaint about interview process, there were no trick-questions, just a call with HR, a call with developer, and then a take-home assignment for a small project I should complete in 4 hours.

The projects I have seen so far: - write a cli web-scraper that downloads images - write a simple HTTP server that accepts files within a certain size limit - write a REST-compliant backend for a simple contacts application - write a url shortener

Sometimes in a language/with tools of your choice, sometimes with language, certain libraries or i.e. backend database specified.

Way I see my programming skill currently, is that most of these for me would be weekend projects, taking me around 12 hours, and what I submit after 4 hours is usually bug-ridden barely working mess, where I often spent almost half of the time debugging weird issues.

What irks me, that 5 years ago, I did much better with these. Maybe I spent too much time as QA/ops/support person? Anybody in similar situation? How did you get to a better level?